ll 

3 


O 


B*< 


FIRE  ASSAYING 


McGraw-Hill  BookCompany 


Electrical  World         The  Engineering  and  Mining  Journal 
Engineering  Record  Engineering  News 

Railway  Age  Gazette*  American  Machinist 

Signal  Engin<?<?r  American  Engineer 

Electric  Railway  Journal  Coal  Age 

Metallurgical  and  Chemical  Engineering  Power 


The  Assayer. — From  Pirotechnia,  Li  Diece  Libri  Delia  Pirotechnia,  by  Vannuccio 
Biringoccio,  Venice,  MDLVIII. — (3rd  edition), 


A  MANUAL 

OF 

FIRE  ASSAYING 


BY 

CHARLES  HERMAN  FULTON,  E.M.,D.Sc. 

PROFESSOR   OP   METALLURGY,    CASE   SCHOOL  OF  APPLIED  SCIENCE. 


SECOND  EDITION 

ENTIRELY  REWRITTEN  AND  ENLARGED 
THIRD  IMPRESSION 


McGRAW-HILL  BOOK  COMPANY,  INC. 
239  WEST  39TH  STREET,  NEW  YORK 

6  BOUVERIE  STREET,  LONDON,  E.  C. 
1911 


COPYRIGHT,  1907,  BY  THE  HILL  PUBLISHING  Co. 


COPYRIGHT,  1911,  BY  MCGRAW-HILL  BOOK  COMPANY. 

AU  rigtUa  reserved 


Printed  and  Electrotype*  by 

The  Maple  Press 

York,  Pa. 


So  Dfs  d&otber 

THIS  BOOK  IS  LOVINGLY  DEDICATED  BT 
THE  AUTHOR 


PREFACE  TO  THE  SECOND  EDITION 


Recent  progress  in  Assaying  has  made  it  desirable  to  add  to 
the  book  and  revise  it. 

The  subjects  of  assay  furnaces,  assay  fluxes,  cupellation  and 
special  methods  of  assay  have  been  added  to. 

The  author  desires  to  express  his  thanks  to  Mr.  W.  J.  Sharwood 
for  additional  material  furnished  for  the  book  and  for  a  search 
of  the  first  edition  for  errors. 

He  also  acknowledges  the  kindness  of  Messrs.  Jay  A.  Carpenter, 
E.  Van  L.  Smith,  0.  A.  Anderson  and  others  in  furnishing  new 
material. 

He  will  appreciate  greatly  the  courtesy  of  assayers,  or  metallur- 
gists who  will  call  his  attention  to  errors  or  pertinent  omissions 
in  this  edition. 

CHARLES  H.  FULTON  . 

CLEVELAND,  OHIO, 

December,  1911. 


ix 


PREFACE  TO  THE  FIRST  EDITION 


The  author  lias  long  recognized  the  need  of  a  work  on  fire 
assaying  tha^  treats  the  subject  from  the  scientific  and  rational 
point  of  view  rather  than  from  that  of  the  "rule  of  thumb." 
Strangely  enough,  this  last  governs  most  modern  works  on  the 
subject.  The  book  is  closely  confined  to  the  subject  of  fire 
assaying,  which  it  treats  in  detail.  The  chapters  on  "  Reduction 
and  Oxidation  Reactions,"  "Crucible  Assay  and  Assay  Slags," 
and  "Cupellation,"  outline  scientifically  the  principles  of  assaying. 
A  large  part  of  these  chapters  is  new  and  some  of  the  material  is 
presented  for  the  first  time.  The  chapter  on  the  "  Errors  in  the 
Assay  for  Gold  and  Silver, "  discusses  the  accuracy  of  the  assay 
in  greater  detail  than  has  been  attempted  heretofore. 

The  author  has  had  experience  with  practically  all  of  the 
methods  of  assay  described  in  the  book;  first  as  a  manipulator, 
then  as  a  teacher,  and  finally  in  charge  of  works.  The  book  is 
intended  for  the  use  of  students  in  technical  schools  and  for  the 
assayer  in  actual  daily  practice  who  may  feel  the  need  of  a 
reference  book. 

The  author  wishes  to  acknowledge  his  indebtedness  to  the 
writers  cited  in  the  text,  especially  to  the  late  Professor  E.  H. 
Miller,  of  Columbia  University,  whose  work  and  personality  has 
ever  been  an  inspiration  to  the  author.  He  also  expresses  his 
thanks  to  Mr.  J.  B.  Read  and  Mr.  Ivan  E.  Goodner,  chemist  and 
assayer  respectively,  for  the  Standard  Smelting  Company,  Rapid 
City,  and  to  Mr.  Frank  Bryant,  his  assistant  at  the  School  of 
Mines,  for  valuable  aid  in  the  testing  of  methods;  to  Professor 
M.  F.  Coolbaugh  for  the  inspection  of  those  chapters  containing 
chemical  equations,  etc.,  and  to  Miss  Ethel  Spayde  and  Miss 
Delia  M.  Haft  for  valuable  aid  received  in  the  preparation  of 
the  manuscript  for  publication.  The  author  also  desires  to  ex- 
press his  appreciation  of  the  courtesy  of  the  Denver  Fire  Clay 
Company  and  of  Ainsworth  &  Son,  Denver,  Colorado,  of  F.  W. 
Braun  and  Company,  Los  Angeles,  California,  and  of  others, 
in  furnishing  photographs  and  electrotypes  of  apparatus  used  in 
the  book. 

CHARLES  HERMAN  FULTON. 

RAPID  CITY,  S.  D., 
April,  1907. 


CONTENTS 

PAGES 
PREFACE vii,  ix 

CHAPTER  I 

ASSAY  FURNACES  AND  TOOLS 1-26 

Assay  Furnaces.  Fuels.  Coal-burning  Muffle-Furnaces. 
Construction.  Dimensions.  Wood-burning  Furnace.  Coke 
Furnace.  Gasolene  Furnace.  Gas  Furnace.  Capacities  of 
Furnaces  and  Cost  per  Assay  for  Fuel  with  Different  Furnaces. 
Muffles.  Furnace  Tools,  Tongs,  etc.  Multiple  Scorifier  Tongs. 
Cupel  Charging  Device.  Molds.  Crucibles.  Scorifiers, 
Roasting  Dishes,  etc.  Crushing  and  Pulverizing  Apparatus. 

CHAPTER  II 

DEFINITIONS;  REAGENTS;  THE  ASSAY  OF  REAGENTS 27-35 

Definitions  of  Assaying.  General  Method  for  the  Determi- 
nation of  Gold  and  Silver.  Reagents  Used  in  Assaying. 
Assay  of  Reagents. 

CHAPTER  III 

SAMPLING 36-41 

Methods  of  Sampling.  Classification  of  Sampling  Methods. 
Principles  of  Sampling.  Sampling  in  Mills  and  Smelters. 
Coning  and  Quartering.  Sampling  by  Alternate  Shovel. 
Sampling  Apparatus  in  the  Assay  Laboratory.  Preparation 
of  the  Assay  Sample.  Control  Assays.  Umpire  Assays. 
Mode  of  Settlement.  Sampling  Lead  and  Copper  Bullion. 

CHAPTER  IV 

WEIGHING;  BALANCES  AND  WEIGHTS 42-52 

The  Assay  Balance.  Method  of  Setting  Up.  Construction. 
Discussion  of  the  Principles  of  the  Balance.  Sensibility. 
Weighing.  Determination  of  Length  of  Balance-arms.  Pulp 
Balance.  Practical  Notes  on  the  Assay  Balance.  Weights. 
The  Assay-ton  System. 

CHAPTER  V 

REDUCTION  AND  OXIDATION  REACTIONS 53-63 

Reduction.     Oxidation.     Reduction  of  Lead  from  Litharge 
by  Argol,  by  Carbon,  by  Flour,  by  Sulphides.     Influence  of 
xi 


Xli  CONTENTS 

PAGES 

Soda.  Reduction  of  Lead  from  Lead  Silicates.  Oxidation  of 
Impurities  by  Niter.  Niter  Reactions  with  Reducing  Agents. 
Reaction  between  Metallic  Lead  and  Niter.  Niter  and 
Carbon.  Niter  and  Pyrite.  Influence  of  Silica  on  the 
Reactions.  Charges  to  Determine  Reducing  and  Oxidizing 
Powers.  Oxidirig  Power  in  Ores. 

CHAPTER  VI 

THE  CRUCIBLE  ASSAY;  ASSAY  SLAGS 64-75 

Nature  of  the  Crucible  Assay.  Influence  of  Fineness  of 
Crushing.  Mode  of  Occurrence  of  Gold  and  Silver,  Physical 
Properties  of  the  Slag,  Chemical  Properties  of  the  Slag. 
Formation  Temperatures  of  Assay  Slags.  Nature  of  Assay 
'  Slags  and  their  Mode  of  Formation.  Influence  on  the 
Assay  of  the  Formation  Temperature.  Constituents  of 
Assay  Slags.  Classification  by  Silicate  Degree.  Influence  of 
Silicate  Degree  and  Different  Bases  on  Formation  Tempera- 
ture. Eutectic  Compositions  of  Slags.  Table  of  Assay  Slags. 
Table  for  the  Calculation  of  Slags.  Example  of  the  Calcula- 
tion of  an  Assay  Slag.  Composition  of  Ores.  Assay  Slags 
Commonly  Made.  Color  of  Slags. 

CHAPTER  VII 

CUPELLATION 76-106 

Object  of  Cupellation.  Bone  Ash  Cupels.  Making  Cupels. 
Magnesia  Cupels.  Cement  Cupels.  Process  of  Cupellation. 
Sprouting  of  Beads.  Freezing-point  Curves  of  Lead-Silver 
and  Lead-Copper.  Temperature  of  Cupellation.  The  "Un- 
covering" and  "Freezing"  of  Lead  Buttons.  "Feathers" 
in  Cupellation.  Influence  of  Impurities  on  the  Process  of 
Cupellation.  Influence  of  Copper.  Influence  of  Tellurium. 
Cupellation  with  Cupels  of  Different  Materia. 

CHAPTER  VIII 

PARTING    107-110 

Ratio  of  Gold  to  Silver  Necessary  to  Part  with  Nitric  Acid. 
Inquartation.  Strength  of  Acid.  Temperature  of  Acid. 
Parting  Devices.  Annealing. 

CHAPTER  IX 

THE  ASSAY  OF  ORES  CONTAINING  IMPURITIES 111-130 

Definition  of  Impurities.  Common  Impurities.  Effect  of 
Impurities  on  Assay.  Production  of  a  Matte.  Effect  of  Silica. 
Effect  of  Soda.  Kind  of  Impurities.  Standard  Methods  of 
Assay.  Roasting.  Niter  Method.  Miller's  Oxide-Slag 
Method.  Perkins'  Excess-litharge  Method.  Niter-Iron 


CONTENTS  Xlll 

PAGES 

Method.  Nature  of  the  Iron-Nail  Fusion.  The  Cyanide 
Method.  Comparison  of  the  Different  Crucible  Methods  of 
Assay.  Scorifi cation.  Scorifiers.  Amount  of  Lead  used. 
Amount  of  Ore.  Process  of  Scorification.  Temperature  of 
Scorification.  Rescorification.  Order  of  Oxidation  of  Metals. 
Applicability  of  Scorification.  Combination  Method.  For 
Blister  Copper.  For  Mattes.  For  Cyanide  Precipitates. 
Precautions  to  be  Observed  in  the  Method. 

CHAPTER  X 

SPECIAL  METHODS  OF  ASSAY 131-159 

Telluride  Ores.  Assay  by  Cripple  Creek  Flux.  By  Excess- 
litharge  Flux.  Behavior  of  Tellurium  in  the  Fusion.  Amount 
of  Tellurium  Present  in  Ores.  Losses  Caused  by  Tellurium. 
Assay  of  Complex  Tellurides  with  Different  Fluxes.  Results 
Obtained.  Assay  of  Copper-bearing  Material.  By  Scorifica- 
tion. Results  Obtained.  Excess-litharge  Method  for  Blister 
Copper.  By  Crucible  Assay.  Assay  of  Material  Containing 
Zinc.  Effect  of  Zinc.  Scorification  for  Zinc  Ores.  Crucible 
Fusion  for  Zinc  Ores.  For  Cyanide  Precipitates.  Assay  of 
Material  Containing  Graphite.  Assay  of  Antimonial  Gold-sil- 
ver Ores.  Arsenical  Ores.  Difficulties  Experienced  with 
these  Ores.  Assay  of  Heavy  Sulphides.  Assay  of  Material 
Containing  Metallic  Scales.  Assay  of  Ores  Containing  Free 
Gold.  The  Assay  of  Slags  and  Cupels.  Amalgamation  Test 
to  Determine  Free  Gold  Present.  Assay  of  Cyanide  Solutions. 
Assay  of  Material  Containing  Metallic  Iron. 

CHAPTER  XI 

ERHOIIS  IN  THE  ASSAY  FOR  GOLD  AND  SILVER 160-173 

Losses  in  the  Cupellation  of  Pure  Silver.  Of  Pure  Gold. 
How  the  Losses  Occur.  Effect  of  Temperature.  Influence  of 
Different  Types  of  Cupels.  Curves  Showing  Cupellation  Loss- 
es. Losses  in  the  Cupellation  of  Gold-Silver  Alloys.  Relative 
Amount  of  Loss  by  Absorption  and  Volatilization.  Slag  Loss 
and  Cupel  Absorption  in  Telluride  Ores.  In  Zinciferous 
Material.  In  Highgrade  Silver  Ores.  In  Cupriferous  Mate- 
rial. General  Discussion  of  Losses.  Other  Errors.  Reten- 
tion of  Lead  or  Copper  in  Beads.  Retention  of  Silver  by  Gold 
after  Parting.  Loss  of  Gold  by  Solution  in  Acid.  Occluded 
Gases.  Error  in  Weighing.  Resume1. 

CHAPTER  XII 

THE  ASSAY  OF  BULLION       174-185 

Classification  of  Bullions.  Assay  of  Lead  Bullion.  The  As- 
say of  Silver  Bullion.  Cupellation  Method,  Preliminary  Assay, 


xiv  CONTENTS 

PAGES 

Check  Assay,  Regular  Assay.  Gay-Lussac  Method  for  Silver 
Bullion.  Standardization  of  Solution,  Apparatus  Required, 
the  Assay,  Calculations.  The  Assay  of  Gold  Bullion  for  Silver 
by  a  Wet  Method.  The  Assay  of  Gold  Bullion.  Preliminary 
Assay,  Check  Assay,  Proof  Alloys.  Regular  Assay.  Prepara- 
tion of  Proof  Gold  and  Silver. 

CHAPTER  XIII 

THE  ASSAY  OF  ORES  AND  ALLOYS  CONTAINING  PLATINUM,  IRIDIUM, 

GOLD,  SILVER,  ETC 180-192 

Difficulty  of  Assay.  Composition  of  Platinum  Nuggets. 
Cupellation  of  Lead  Containing  Platinum,  etc.  Appearance  of 
Cupeled  Bead.  Action  of  Acids  on  Metals  Contained  in  the 
Platinum-Silver  bead.  Nitric  Acid.  Sulphuric  Acid,  Nitro- 
hydrochloric  Acid.  Methods  of  Assay  to  Obtain  Lead  Button. 
For  Ores  Containing  Metallic  Grains.  For  Alloys.  Method  of 
Assay  by  Parting  Silver-Platinum  Alloys  in  Sulphuric  Acid, 
etc.  Method  of  Assay  by  Dissolving  Lead  Button  in  Nitric 
Acid.  Results  Obtainable. 

CHAPTER  XIV 

THE  ASSAY  OF  TIN,  MERCURY,  LEAD,  BISMUTH  AND  ANTIMONY.        193-201 

General  Remarks  on  the  Fire  Assay  for  Base  Metals.  The 
Assay  of  Tin  Ores.  Causes  of  Loss  in  the  Assay.  Prepara- 
tion of  the  Ore  for  Assay.  The  Cyanide  Method.  The  Ger- 
man Method.  Results  Obtainable.  The  Mercury  Assay.  Ap- 
paratus Required.  The  Conduct  of  the  Assay.  Results 
Obtainable.  Assay  of  Lead  Ores.  Inaccuracies  of  it.  Lead 
Flux  Method.  Soda-Argol  Method.  Cyanide  Method.  Reac- 
tions in  the  Assay.  Results  Obtainable.  The  Assay  of 
Antimony  and  Bismuth  Ores. 

APPENDIX 202-205 

Conversion  Table  for  Weights.  Table  of  Assay  Valuations. 
Table  of  Fineness  of  Bullion  and  Alloys  of  Precious  Metals. 
Table  of  Volume  and  Weight  of  Fine  Gold  and  Silver. 

INDEX    .  .   207-219 


LIST  OF  ILLUSTRATIONS 

FIG.  PAGE 

1.  Two-Muffle  Furnace.     Perspective  View 2 

2.  Two-Muffle  Furnace.     Cross  Section 3 

3.  Two-Muffle  Furnace.     Longitudinal  Section 4 

-4.  Three-Muffle  Furnace.     Cross-Section 5 

5.  Three-Muffle  Furnace.     Longitudinal  Section 6 

6.  Oil-burning  Furnace.     Cross-Section 7 

7.  Wood-Burning  Muffle-Furnace.     Cross-Section 8 

8.  Wood-Burning  Muffle-Furnace.     Longitudinal  Section 9 

9.  Wood-Burning  Muffle-Furnace 10 

10.  Muffle-Furnace  for  Burning  Coke 11 

11.  Combination  Muffle  and  Pot  Furnace 12 

12.  Combination  Muffle  and  Pot  Furnace 12 

13.  Gasolene  Furnace  Apparatus •.  1.3 

14.  The  Gary  Gasolene  Burner 1.3 

15.  Gasolene  Tank  and  Pump  Apparatus 14 

16.  Gasolene- Burning  Crucible  Furnace 15 

17.  Gasolene- Burning  Muffle-Furnace .    .  16 

18.  Case-Burner . 17 

19.  Case  Gas- Burner 18 

20.  Muffle  Furnace  with  Special  Supports ......  19 

21.  Gas-Burning  Muffle-Furnace 20 

22.  Crucible  Tongs.     Undesirable  Model 21 

23.  Crucible  Tonga      21 

24.  Cupel  Tongs 21 

25.  Scorifier  Tongs ' 21 

26.  Scorifier  Tongs 21 

27.  Multiple  Scorifier  Tongs  (Keller) 22 

28.  Multiple  Scorifier  Tongs  (Keller) "...  22 

29.  Cupel  Charging  Device  (Keller) 22 

30.  Crucible  Pouring  Molds 23 

31.  Cupel  Tray 23 

32.  Fire-Clay  Annealing-Cup  Tray 23 

33.  Fire-Clay  Crucibles 24 

34.  Scorifiers 25 

35.  Buck  Board  and  Muller      25 

36.  Buck  Board  Brushes '.    .    .  26 

37.  Jones  Riffle  Sampler 40 

38.  Umpire  Ore  Sampler 41 

39.  Diagram  of  Assay  Balance 43 

40.  Pulp  Balance 48 

41.  Assay  Button  Balance 49 

42.  Non-Column  Type  of  Assay  Balance 50 

XV 


XVI  LIST  OF  ILLUSTRATIONS 

FIG.  PAGE 

43.  Platinum  Assay  Weights     ...-..-. 50 

44.  Assay-Ton  Weights      . 51 

45.  Gram  Weights 51 

46.  Freezing-Point  Curve.     Rhodonite-Hypersthene 67 

47.  Cupel  Machine 78 

48.  Freezing-Point  Curve  of  Lead-Silver 85 

49.  Freezing-Point  Curve  of  Lead-Copper 85 

50.  Temperature-Curve,  Air  in  Muffle  and  Cupelling-Lead      87 

51.  Temperature-Curve  During  Cupellation  Showing  Uncovering  and 
"Freezing"  of  Button 89 

52.  Temperature-Curve,  Cupellation  of  Lead-Silver 95 

53.  Temperature-Curve,  Cupellation  of  Lead-Silver 96 

54.  Temperature-Curve,  Cupellation  of  Lead-Silver 96 

55.  Parting  Bath 109 

56.  Parting  Flasks 109 

57.  Homestake  Agitator 155 

58.  Curve  Showing  Silver-Losses  in  Cupellation 163 

59.  Jeweller's  Rolls 182 

60.  Apparatus  Required  for  Mercury  Assay 199 


A  MANUAL  OF  FIRE  ASSAYING 


CHAPTER  I 
ASSAY  FURNACES  AND  TOOLS 

FURNACES. — The  furnaces  used  in  assaying  are  many  in  design, 
varying  mainly  with  the  kind  of  fuel  used.  The  furnaces  are 
classified  as  follows:  (1)  Pot  furnaces,  in  which  the  assay  is  in 
direct  contact  with  the  fuel;  (2)  Muffle-furnaces,  in  which  a 
muffle  or  receptacle  containing  the  assay  is  externally  heated. 

As  the  muffle-furnace  is  practically  essential1  for  carrying  on 
the  operations  of  scorification  and  cupellation,  and  crucible 
fusions  can  be  made  satisfactorily  in  the  muffle  if  it  be  large 
enough,  muffle-furnaces  have  largely  replaced  pot  furnaces  for 
general  assaying.  In  general,  they  are  cleaner,  more  easily 
operated,  better  controlled  as  to  temperature,  and  if  large  enough 
are  of  great  capacity,  which  makes  them  especially  desirable  for 
smelter,  mill  and  mine  assay  offices,  where  frequently  a  great 
number  of  assays  are  performed  daily.  The  choice  of  fuel  for 
heating  the  furnaces  is  usually  dependent  on  locality.  Bitumi- 
nous and  lignite  coal,  coke,  anthracite,  crude  oil,  gasolene  or 
kerosene,  wood,  fuel  and  illuminating  gas  are  all  used.  Of 
these,  coke  and  anthracite  are  the  fuels  least  desirable  for  muffle- 
furnaces,  for  burning  without  flame  they  must  surround  the 
muffle.  This  makes  the  firing  difficult,  requiring  considerable 
attention.  The  best  fuel,  usually  also  the  most  easily  obtainable, 
is  bituminous  or  good  lignite  coal,  yielding  a  long  or  reasonably 
long  flame.  One-,  two-  and  three-muffle  furnaces,  constructed 
of  fire-clay  tiling,  fire-brick,  and  common  hard  brick,  tightly 
bound  with  stays  and  rods,  are  in  common  use,  and  for  general 
utility,  where  much  work  must  be  performed,  are  very  desirable. 

1  "Koenig's  Furnace,"  in  Trans.  A.  I.  M  E.,  XXVIII,  271.  This  furnace  is  practically 
a  pot  furnace  fired  by  gasolene,  and  with  an  air  blast  can  be  used  to  scorify  and  cupel  with- 
out a  muffle. 

1 


A    MANUAL    OF    FIRE    ASSAYING 


Coal,  Coke  and  Oil  Furnaces. — Fig.  1  shows  such  a  two- 
muffle  furnace  in  perspective,  and  Figs.  2  and  3,  in  cross-section. 
The  essential  parts  of  the  furnace,  as  the  tiling,  A,  B,  L,  K,  etc., 
can  be  readily  purchased,  although  the  interior  of  the  furnace 
may  also  be  built  of  fire-brick.  The  tiling  furnace,  however, 
is  more  easily  set  up  and  is  more  durable.  In  the  design  of  the 
soft-coal  furnace,  the  essential  dimensions  are:  area  of  fire-grate; 
distance  from  the  grate  to  the  bottom  of  the  lower  muffle;  the 
"fire  space,"  i.e.,  the  distance  between 
muffles  and  the  side  and  end  walls  of 
the  furnace,  and  between  the  top  of  the 
upper  muffle  and  the  roof  of  the  furnace, 
Jir '  i  '  i  EjE  ~i~H-  giving  the  proper  space  for  combustion 
E£-  of  the  gases.  These  dimensions  depend 
upon  the  nature  of  the  coal.  In  Figs. 
2  and  3  the  grate  dimensions  are  17.25 
X 2 1.0  in.;  distance  from  grate  to  lower 
muffle,  18  in.;  fire  space,  2.5  in.;  external 
dimension  of  muffle,  19  in.  long,  12.25  in. 
wide,  7.75  in.  high.  The  flue  area  should 
be  from  one-sixth  to  one-eighth  of  the 
grate  area.  The  flue  is  best  placed  for- 
ward of  a  line  through  the  center  of  the 
muffles  to  get  the  full  sweep  of  the  flame 
around  them,  although  this  arrange- 
FIO.  I.-TWO-MOFFLE  FOBNACB.  ment  wifh  poOr  draft,  is  apt  to  cause 

Perspective  view.  . 

smoky  muffles. 

The  walls  of  the  furnace  are  thick  (13  in.)  to  prevent  radiation. 
The  front  of  the  furnace  above  the  muffle  is  arched.  The  arch 
tiling  has  in  it  a  duct,  leading  to  the  flue,  to  carry  off  lead  fumes. 
The  muffles  are  supported  by  two  sets  of  tiles,  placed  into  the 
side  walls  and  sometimes  by  an  additional  set  in  the  rear  end 
wall.  These  tiles  frequently  prove  weak,  and  in  falling  away 
leave  the  muffle  without  support,  causing  it  to  be  short  lived. 
The  supports  are  best  made  in  such  shape,  of  two  pieces,  that 
they  will  join  under  the  center  line  of  the  muffle  and  arch  over, 
supporting  each  other.  The  writer  has  used  supports  of  this 
type,  which  were  perfectly  satisfactory  and  increased  the  life 
of  the  muffles  greatly.  A  furnace  of  the  kind  described  has  a 
capacity  of  25  to  30  fusions  (20-gram  crucible)  per  hour,  including 
the  necessary  cupellations.  If  the  fusions  are  made  in  30-gram 


ASSAY   FURNACES    AND    TOOLS 


3 


crucibles  or  in  2.5-in.  scorifiers,  the  capacity  is  from  20  to  24. 
With  good  draft,  this  furnace  burns  from  37  to  47  Ib.  of  coal 
per  hour,  which  at  $7.00  per  ton,  makes  the  cost  per  assay  for 
fuel  amount  to  from  0.80  cents  to  1.00  cent,  when  assaying  con- 
tinuously and  somewhat  more  when  the  furnace  is  not  charged 
to  its  maximum  capacity.  With  a  good  grade  of  coal  (6500 


FIG.  2. — TWO-MUFFLE  FUBXACE      Cross-section. 

to  7500  calories),  a  maximum  temperature  of  1150°  to  1200° 
C.  can  be  obtained  in  this  furnace  after  4  hours  firing.  Figs.  4 
and  5  show  a  three-muffle  furnace  of  similar  type. 

Coal  furnaces  may  also  be  readily  modified  to  burn  crude  oil. 
This  can  be  done  by  placing  tiling  in  the  fire-box,  and  making 
the  necessary  pipe  and  burner  connections. 1 

IF.  C.  Bowman,  "Crude  Oil  for  Fire  Assaying,"  Proc.  Colo.  Sci  Soc.,  VII,  341. 


4  A   MANUAL    OF    FIRE   ASSAYING 

Fig.  6  shows  such  a  furnace.  The  burner  is  a  £  in.  pipe 
connected  by  a  T  to  the  oil  line,  also  a  £  in.  pipe.  A  £  in. 
steam  pipe  passes  into  the  burner  pipe  at  the  rear  through  a 
packing  nut  which  permits  of  the  adjustment  of  the  distance 
between  the  nozzle  of  the  burner  pipe  and  the  nozzle  of  the  steam 
pipe.  By  varying  this  distance  the  flow  of  oil  may  be  affected 


fl4  Sheet  Steel  Plate 


Fia.  3. — TWO-MUFFLE  FURNACE.     Longitudinal  section. 

independent  of  the  steam  and  oil  inlet  valves.  The  nozzle  of 
the  burner  pipe  is  an  \  in.  hole  and  that  of  the  steam  pipe  an 
$  in.  hole. 

The  grate  bars  in  the  furnace  are  covered  with  fire  brick  as 
shown  in  the  illustration.  The  placing  of  the  fire  brick  is  of 
importance  as  the  successful  working  of  the  furnace  is  dependent 
upon  their  position. 

In  starting  the  fire  a  piece  of  oiled  waste  is  lighted  in  the  fire 
box  just  back  of  the  burner.  When  the  waste  is  burning  well, 
oil  and  steam  are  turned  on  simultaneously.  The  oil  and  steam 
valves  are  then  set  to  give  the  proper  flow.  Plenty  of  waste 
should  be  used  to  furnish  a  blaze  until  the  fire  box  is  hot  enough 


ASSAY    FURNACES   AND    TOOLS  5 

to  ignite  the  oil,  otherwise  explosions  are  apt  to  occur.  The 
steam  used  should  be  dry,  and  to  insure  this  the  steam  pipe 
leading  to  the  burner  may  be  passed  around  the  flue  as  shown 
in  the  figure.  The  valve  DV  at  the  end  of  the  steam  line  is 
kept  slightly  open  during  working  to  permit  the  escape  of  water 
of  condensation.  A  small  steam  coil  may  also  be  placed  in  the 
oil  tank  to  keep  the  oil  more  fluid.  The  furnace  may  be  heated 
to  a  red  heat  15  to  20  minutes  after  starting.  The  furnace  has 
a  capacity  of  25  to  30  assays,  including  cupellations  in  If  to  2 


FIG.  4. — THREE-MUFFLE  FURNACE.     Cross-section. 

hours  and  50  to  60  assays  in  3  hours.  The  amount  of  oil  used 
varies  from  4.2  to  5.3  gals,  per  hour.  With  oil  at  8£  cents  per 
gal.  the  cost  per  assay  for  fuel  is  2.2  cents  to  2.8  cents.  Figs. 
7  and  8  show  a  wood-burning  muffle-furnace.  In  some  districts 
wood  is  the  only  available  cheap  fuel.  If  the  fire-box  and  fire 
spaces  are  properly  designed  (i.e.,  of  larger  size  than  in  the 
coal  furnace)  and  a  deep  bed  of  fuel  is  provided  for  (i.e.,  the 
distance  from  the  grate  surface  to  the  bottom  of  the  fire-door 
is  from  8  to  10  in.),  sufficient  temperature  for  ordinary  assaying 
can  be  attained  in  this  type  of  furnace.  Almost  any  wood  may 
be  used. 


6 


A   MANUAL    OF    FIRE    ASSAYING. 


In  the  furnace  shown  in  Figs.  7  and  8,  the  grate  is  18  in. 
wide  and  26  in.  long;  the  distance  from  the  grate  bars  to  the 
bottom  of  the  muffle  is  26  in.  and  the  fire  space  is  2.5  in.  wide 
at  the  sides  and  3.5  in.  at  the  top.  Fig.  9  shows  a  wood  burning 
furnace  of  somewhat  different  construction.1  With  pinon  pine 
or  fir  wood  at  $4.50  per  cord,  the  cost  of  fuel  is  65  cents  for  a 
daily  run  of  30  to  40  assays.  With  a  poor  grade  of  wood  at 


FIG  5. — THREE-MUFFLE-FUHNACE      Longitudinal  Section. 

$6.50  per  cord,  in  another  instance,  30  assays  cost  93  cents,  or 
3.1  cents  per  assay,  including  cupellations. 

Coke1  and  anthracite  muffle-furnaces  when  used  are  usually 
smaller,  although  large  furnaces  may  be  specially  designed  and 
built  of  the  general  type  of  the  coal  furnaces  described. 

Fig.  10  shows-a  small  coke  or  anthracite  furnace.  The  fuel 
is  fed  in  at  the  top  and  kept  well  heaped  around  the  muffle.  A 
furnace  of  the  kind  shown  in  Fig.  10  will  consume  from  32  to  38 
Ib.  of  coke  per  hour,  according  to  draft.  With  a  muffle  11  X16 
X7  in.,  10  assays  per  hour,  including  cupellation,  can  be  made 

E.  H.  Nutter,  M in.  and  Sci.  Press,  XCII,  329,  and  Louis  Janin,  Jr.,  Eng  and  Min.  Jour., 


ASSAY    FURNACES    AND    TOOLS 


FIG.  6. — OIL-BURNING  FURNACE.     Cross-section. 


s 


A    MANUAL    OF    FIRE    ASSAYING 


at  a  cost  of  1.7  cents  per  assay  with  coke  at  $10  per  ton.     Char- 
coal may  be  used  in  this  furnace  in  place  of  coke. 

Fire-clay  muffles  for  furnaces  are  made  in  varying  sizes  and 
shapes.  The  best  shape  for  general  use  is  one  of  nearly  rect- 
angular cross-section,  with  but  a  slightly  arched  top.  The  lar- 
gest muffles  ordinarily  used  are  19  in.  long,  14.5  in.  wide  and 
7.75 'in.  high  (outside  dimensions).  Muffles  19  in.  long,  12  in. 
wide  and  7.75  in.  high  are  very  common  in  coal  furnaces.  The 
muffles  have  two  holes  in  the  rear  end  to  induce  an  air  draft 
through  them. 


Fia.  7. — WOOD-BURNING-MUFFLE-FURNACE      Cross-section. 

The  method  of  support  of  the  muffle  in  the  furnace  has  great 
bearing  on  the  life  of  the  muffle.  Muffles  inadequately  sup- 
ported soon  crack  and  fall  to  pieces.  It  is  perhaps  better  to 
support  muffles  by  one  substantial  rather  broad  support  near 
the  middle  and  across  the  whole  bottom,  and  by  resting  the 
front  end  on  the  furnace  wall  and  the  rear  end  on  two  replacable 
clay  supports,  than  to  have  more  numerous  supports  extending 
short  distances  only  beyond  the  walls  on  the  bottom.  Muffles 
should  be  stored  in  a  dry  warm  place  to  prevent  their  absorbing 
moisture  and  when  new  muffles  are  placed  in  a  furnace,  it  should 
be  fired  lightly  with  wood  chips  for  an  hour  to  anneal  the  muffles. 


ASSAY    FURNACES    AND    TOOLS 


9 


before  heavy  firing  is  begun.  The  spilling  of  slag  and  lead  in  the 
muffle  rapidly  leads  to  corrosion  and  softening  of  the  bottom 
and  consequent  destruction.  To  avoid  this  deterioration  in 
part,  muffle  bottoms  should  be  covered  with  a  layer  about  \  in. 
thick  of  bone  ash,  silica  sand,  or  Portland  cement,  to  act  as  an 
absorbent.  Muffles  are  also  subject  to  destruction  from  the 
fluxing  action  of  the  ashes  of  the  fuel  burnt  on  the  grate.  Ashes 
high  in  iron  oxide  are  the  worst  in  this  respect. 

In  setting  muffles,  it  is  essential  for  the  attainment  of  the 
best  heating  conditions  to  thoroughly  lute  up  the  space  around 


FIG.  8. — WOOD-BURNING  MUFFLE-FUBNACE.     Longitudinal  Section. 

the  edge  of  the  muffle  and  the  arch  opening  in  the  front  of  the 
furnace  into  which  it  fits.  If  this  space  be  of  considerable  size, 
it  is  best  filled  in  roughly  with  chips  of  broken  crucibles,  etc., 
before  applying  the  luting  material.  Luting  material  may  be 
of  fire  clay  and  crushed  fire  brick  or  crucibles,  one-fourth  of  the 
former  to  three-fourths  of  the  latter,  mixed  with  sufficient  water 
to  make  a  plastic  mass.  The  fire  brick  may  be  crushed  to  pass 
an  eight  mesh  screen.  Raw  fire  clay  has  too  great  a  shrinkage 
to  be  used  alone.  One-quarter  fire  clay,  one-quarter  shredded 
asbestos,  and  one-half  crushed  fire  brick  makes  a  good  luting 
material. 


10 


A   MANUAL    OF    FIRE   ASSAYING 


Figs.  11  and  12  represent  a  combination  of  crucible  pot  furnace 
and  muffle  furnace,  such  as  is  used  in  England.  The  furnace 
may  be  built  of  ordinary  fire  brick  and  is  dimensioned  in  such  a 
manner  as  to  avoid  cutting  brick  as  much  as  possible.  It  is 
fired  by  coke.  The  two  crucible  furnaces  S  connect  with  the 
main  flue,  9X9  in.-  in  size,  by  the  flues  N.  The  crucible  furnace 
nearest  the  muffle  also  connects  with  the  muffle  furnace  by  the 
extra  flue  0.  By  these  means  the  hot  gases  of  combustion  may 


FIG.  9. — WOOD-BURNING  MUFFLE-FURNACE. 

be  diverted  to  the  muffle  furnace,  instead  of  directly  to  the  stack. 
The  pot  furnaces  are  14x14  in.  in  section,  and  on  account  of 
the  sloping  top,  are  17  in.  deep  at  the  front  and  23  in.  at  the 
back.  The  top  of  the  furnace  is  best  made  of  a  sheet  steel  plate 
cut  as  required.  The  doors  are  made  of  two  tiles,  each  20x10 
X4  in.,  held  together  by  two  pieces  of  1.5  in.  channel  iron  clamped 
by  two  0.5  in.  rods.  To  the  ends  of  these  rods,  four  3  in.  iron 
wheels  are  fastened  on  which  the  doors  run.  The  tiles  are 
secured  in  the  frame  in  such  a  manner  that  there  is  a  clearance 


ASSAY    FURNACES    AND    TOOLS 


11 


of  0.25  in.  above  the  furnace  top  to  freely  move  the  doors.  Each 
furnace  has  8  grate  bars  of  1  in.  square  wrought  iron  resting  at 
the  ends  on  two  similar  bars,  placed  on  the  brickwork.  The 
bars  are  14  in.  long  except  the  two  center  ones  which  are  18  in. 
long,  and  may  be  withdrawn  through  the  opening  G  for  dumping 
the  fire. 

There  are  four  muffles  H,  in  the  furnace,  15X9X6  in.  outside 
measurements.  The  lower  muffle  and  perhaps  the  next  upper 
one  may  be  used  for  scorification  and  cupellation.  The  other 
two  muffles  will  not  heat  to  a  high  enough  temperature  for  any- 
thing except  annealing  and  roasting.  The  muffles  rest  at  the 


FIG.  10. — MDFFLE-FURNACE  FOR  BURNING  COKE. 

back  and  sides  on  the  ends  of  bricks  cut  to  a  level  as  shown  and 
projecting  from  the  furnace  body.  At  the  front  they  lie  flush 
on  1.5  in.  angle  irons  A. 

The  furnace  is  built  with  the  front  entirely  open;  the  grate 
bars  are  the  same  as  described  for  the  crucible  furnace.  When 
the  muffles  have  been  placed  in  position  the  space  around  the 
front  of  the  muffles  is  rilled  with  a  mixture  of  fire  clay  and  silicate 
of  soda  to  a  depth  of  3  in.  A  strong  solution  of  silicate  of  soda 
or  "water  glass"  is  mixed  with  3  times  its  weight  of  water  until 
homogeneous.  This  solution  is  then  mixed  with  fire  clay  to  a 
stiff  paste;  usually  1  part  of  solution  is  required  for  7  parts  of 
fire  clay.  The  mixture  usually  contracts  on  heating  and  shrinks 
away  at  the  edges.  These  cracks  then  have  to  be  filled  again. 


12 


A    MANUAL    OF    FIRE    ASSAYING 


The  top  of  the  muffle  furnace  is  covered  with  tile  laid  in  1.5  in. 
angle  irons  A'.  The  flue  V  from  the  muffle  furnace  into  the 
stack  is  12  X  3  in.  in  size.  The  draft  of  the  furnaces  is  controlled 
by  placing  sheet  iron  plates  in  front  of  the  ash  pit  doors. 


End  Elevation 
FIG.  11. — COMBINATION-MUFFLE  AND  POT-FURNACE. 


Fio.   12. — COMBINATION-MUFFLE  AND  Pi 


'OT-FTONACE. 


Gasolene -fired  Furnaces. — Furnaces  of  this  type  are  in  com- 
mon use,  and  for  small  offices,  where  the  pressure  of  work  is 
not  great,  they  afford  a  convenient  and  cheap  method  of  operation. 


ASSAY    FURNACES    AND    TOOLS  13 

Gasolene,  on  account  of  ease  of  transportation  and  great  calorific 
power,  is  also  employed  in  out-of-the-way  districts  for  extensive 
daily  work.  Where  coal  is  reasonably  cheap,  not  above  $6.50 
per  ton,  gasolene  at  30  cents  per  gal.  cannot  compete  with  it  in 


FIG.  13. — GASOLENE  FURNACE  APPARATUS. 

large  offices  or  schools,  where  the  assay  furnaces  are  operated 
continuously  for  the  greater  part  of  the  day. 

Fig.  13  shows  a  gasolene  furnace  apparatus.  The  furnace, 
divided  into  crucible  and  muffle  compartments,  is  made  of  fire- 
clay tiling,  bound  with  sheet  iron.  It  is  heated  by  a  brass  and 
copper  burner,  provided  with  a  generating  device.  The  burners 


FIG.  14. — THE  CAKY  GASOLENE  BURNER. 


are  made  in  varying  sizes  to  suit  different  furnaces.  The  gaso- 
lene is  stored  in  a  steel  tank,  of  5  or  10  gal.  capacity,  provided 
with  an  air  pump  to  furnish  pressure.  A  pressure  gauge  is  at- 
tached to  the  tank.  Generally,  0.25-  to  0.375-in.  piping  joins 
the  tank  and  the  burner.  The  burner  and  piping  are  connected 


14  A   MANUAL    OF   FIRE    ASSAYING 

by  a  special  universal  joint,  so  that  the  burner  can  be  swung  into 
and  out  of  position.  The  burner  (if  the  Gary)  should  fit  tightly 
against  the  fire-clay  ring  or  boss  in  the  opening  of  the  furnace,  so 
that  all  the  air  for  the  combustion  of  the  gasolene  is  drawn  in 
through  the  burner  tube.  To  insure  tight  joints,  glue  or  soap, 
or  shellac,  not  white  or  red  lead,  must  be  used  in  the  screw  con- 
nections. The  gasolene  is  fed  to  the  burner  under  a  pressure 
of  10  to  20  lb.,  though  for  special  purposes  higher  pressures  are 
used. 


FIG.  15. — GASOLENE  TANK  AXD  PUMP  APPARATUS. 

Fig.  14  shows  a  detailed  view  of  the  Gary  burner.  The  upper 
valve  controls  the  main  gasolene  supply,  and  the  lower  one 
controls  the  generator.  The  burner  is  heated  by  the  generator, 
so  that  the  gasolene  issuing  from  the  main  needle-valve  is 
vaporized,  and  in  its  passage  to  the  furnace  draws  in  air  through 
the  burner  tube,  the  mixture  igniting  and  burning  at  the  mouth 
of  the  burner  in  the  hot  furnace.  Burners  are  listed  by  the  diam- 
eter of  their  tubes.  Five  sizes  are  made,  from  1.25  to  2.25  in., 
each  size  varying  by  0.25  in. 


ASSAY   FURNACES    AND    TOOLS  15 

Fig.  15  shows  the  tank  and  pump  apparatus.  It  is  best  to 
place  this  at  a  considerable  distance  from  the  furnace,  in  order 
to  avoid  accidental  explosions.  Fig.  16  shows  a  crucible  furnace, 
and  Fig.  17  a  large  gasolene  muffle-furnace.  The  writer  has 
attained  a  temperature  of  1350°  C.  in  small  gasolene  furnaces, 
such  as  Fig.  16  represents,  and  1250°  C.  in  large  furnaces,  as 
represented  by  Fig.  17.  By  a  special  construction  of  furnace, 
with  graphite  muffle  and  heavy  insulation  against  radiation, 
with  good  draft,  the  writer  has  attained  (for  metallurgical  experi- 
mentation) temperatures  of  1500°  to  1530°  C.,  after  three  hours, 


Fio.  16. — GASOLENE-BURNING  CRUCIBLE  FURNACE. 

with  a  2-in.  gasolene  burner  as  shown  in  Fig.  14,  with  gasolene  at  a 
pressure  of  55  Ib.  and  a  consumption  of  1.53  gal.  per  hour.  A 
2-in.  Gary  burner,  under  10  Ib.  pressure,  will  consume  from 
0.65  to  0.75  gal.  per  hour.  A  No.  31  Gary  combination  furnace, 
holding  at  a  charge  in  the  crucible  compartment  six  20-gram 
crucibles  and  having  a  muffle  7x10.5x4.5  in.  in  size,  has  a 
capacity  of  10  fusions  per  hour,  including  cupellation.  With 
gasolene  at  30  cents  per  gallon,  the  cost  of  fuel  per  assay  is 
2.25  cents. 

Fig.  18  shows  the  Case  burner  for  gasolene  or  similar  distillate. 
When  in  the  proper  position  it  is  inverted,  i.  e.,  the  preheating 
system  is  at  the  top  instead  of  at  the  bottom  as  in  the  Gary 


16 


A    MANUAL    OF    FIRE    ASSAYING 


burner.  The  generator  or  boss  in  which  the  gasolene  is  vapor- 
ized is  cast  in  one  piece  with  the  mixing  chamber  which  is  in  the 
form  of  a  truncated  cone  and  very  much  shorter  than  in  other 
gasolene  burners.  The  burner  is  smaller  and  more  compact 
than  the  ordinary  burner  of  the  same  capacity.  The  fact 
that  it  is  inverted  permits  the  gas  formed  from  the  gasolene 


FIG.  17. — GASOLENE-BURNING  MUFFLE-FURNACE. 

in  the  generator  to  pass  freely  upward  to  the  valves.  The 
valves  are  of  special  design.  Ordinarily  the  needle  valve 
is  used  in  burners  of  this  type,  that  is,  a  pointed  hard  steel 
needle  works  in  the  circular  valve  orifice,  making  an  annular 
opening  for  the  escape  of  the  gas.  This  annular  opening  varies 
in  dimensions  according  to  the  position  of  the  needle,  and  may 
be  closed  completely  by  screwing  the  needle  up  as  far  as  it  will 


ASSAY   FURNACES    AND    TOOLS 


17 


go.  With  use,  the  tendency  of  the  needle  is  to  enlarge  the  valve 
orifice  and  cause  increased  consumption  of  gasolene.  The  valve 
of  the  Case  burner  is  closed  by  the  valve  seat  meeting  a  shoulder 
on  the  valve  stem,  both  planed  surfaces.  The  opening  for  the 
flow  of  gas  is  annular  as  before,  but  the  end  of  the  blunt  valve 
pin  does  not  close  the  valve.  The  burner  is  made  of  phosphor 
bronze,  and  operates  best  under  a  pressure  of  from  40  to  50  Ib. 

Fig.  19  shows  a  gas  burner  for  assay  furnaces.  The  air  supply 
is  controlled  by  the  butterfly  valve  A.  The  gas  issues  from  the 
circular  opening  D  and  mixes  with  the  air  from  the  annular 


FIQ.   18. — CASE-BURNER.     A,  Nipple  containing  gravel  for  straining  gasolene; 
B,  Generator;  C,  Valve. 

opening  C,  for  combustion.  The  gas  flow  is  regulated  by  the 
cock  G.  The  burner  is  provided  with  the  pilot  tube  B  to  ignite 
the  gas  in  starting  the  burner. 

Fig.  20  shows  a  Case  gasolene,  oil,  or  gas  fired  muffle  furnace 
of  new  design.  It  is  provided  with  a  heating  chamber  the 
features  of  which  are  first — a  set  of  fire-clay  blocks  so  con- 
structed as  to  form  channels  or  flues  under  the  muffle  to  direct 
the  flame  and  hot  products  of  combustion,  insuring  a  uniform 
heat  distribution  and  acting  as  a  firm  support  for  the  muffle; 
and  second — sets  of  vertical  ribbed  channels  or  flues  in  the  side 
walls  to  guide  the  hot  gases  and  accomplish  an  even  distribution 
of  the  heat.  The  channels  between  the  ribs  are  wider  near  the 
front  than  at  the  rear  of  the  furnace  in  order  to  lessen  friction 
to  the  gases  in  this  part,  thus  causing  the  heating  of  the  front 
of  the  muffle  uniformly  with  the  rest  of  it. 


18  A   MANUAL    OF    FIRE    ASSAYING 

The  fire  clay  blocks  or  "muffle  heaters"  may  be  readily  re- 
placed by  new  ones  when  necessary. 

Gas  Furnaces. — Where  municipal  illuminating  gas  or  other 
gaseous  fuel  is  available,  gas-fired  furnaces  are  convenient  and 
cheap  of  operation.  The  Reichhelm  furnace  (American  Gas 
Furnace  Company)  is  frequently  used.  The  furnaces  require 
air  at  low  pressure,  which  is  mixed  with  gas  in  proper  proportion 
before  it  enters  the  furnace  through  the  several  burners.,  The 


FIG.  19. — CASE  GAS-BURNEB,  AIR  FURNISHED  BY  Low  PRESSURE  BLOWER. 

proportion  of  gas  to  air  is  controlled  by  valves.  Fig.  21  shows 
the  furnace.  Gas  furnaces  permit  of  close  control  of  heat  and  are 
desirable  for  accurate  temperature  work. 

FURNACE  TOOLS. — Convenient  tools  are  necessary  for  the  hand- 
ling of  crucibles,  scorifiers  and  cupels.  The  features  essential 
in  these  tools  are  that  they  be  light,  grasp  the  crucible,  etc., 
firmly,  with  no  danger  of  tipping,  and  take  up  little  room  in  the 
furnace.  As  an  illustration  of  a  tool  deficient  in  these  qualities 
and  therefore  undesirable,  Fig.  22  is  given.  This  shows  a  pair 
of  crucible  tongs  designed  to  grasp  the  body  of  the  crucible.  It 
cannot  be  handled  in  a  muffle  full  of  crucibles,  owing  to  the  space 
it  takes  up  in  opening.  Fig.  23  shows  a  pair  of  crucible  tongs  to 
grasp  the  sides  of  the  crucible,  and  operating  in  little  space. 


ASSAY   FURNACES    AND    TOOLS  19 

Fig.  24  shows  two  types  of  cupel  tongs.     Fig.  25  shows  a  good 
form  of  scorifier  tongs,  and  Fig.  26  another  form. 

For  large  offices  where  much  work  must  be  quickly  accom- 
plished, special  forms  of  tools  may  be  used.  Figs.  27  and  28 
show  a  multiple  tongs1  for  scorifiers.  This  apparatus  will  handle 
25  scorifiers,  practically  a  muffleful  at  one  time.  It  is  composed 
of  quintuple  tongs,  corresponding  to  the  five  longitudinal  rows 
of  scorifiers  in  the  muffle.  The  lower  part  of  each  pair  of  the 
tongs  consists  of  a  fork  on  which  the  scorifiers  rest,  and  one  of 


PlG.     20. MUFFLE-FURXACE  WITH  SPECIAL  SUPPORTS. 

whose  prongs  is  rectilinearly  extended  through  two  bearings  in  a 
frame  and  held  in  position  by  collars.  This  extension  is  free  to 
revolve  on  the  bearings,  and  it  is  the  axis  of  rotation  of  the  tongs. 
To  each  of  them  is  attached,  at  a  right  angle,  a  lever  extending 
upward  at  45°,  and  all  the  levers  are  connected  by  slotted  joints 
to  a  cross-rod.  Therefore  if,  by  means  of  a  crank  fastened  to 
the  end  of  one  of  the  extended  prongs,  one  of  the  forks  is  turned 
and  the  scorifiers  tilted  to  the  desired  angle,  the  others  rotate  to 
the  same  extent.  The  center  of  gravity  of  the  scorifiers  lies  to 

1  Edward  Keller,  "Labor-Saving  Appliances  in  the  Works  Laboratory,"  in  Trans.  A.  I.  M. 
E.,  XXXVI,  3,  and  Bui.  44,  633,  Aug.,  1910. 


20  A    MANUAL    OF    FIRE    ASSAYING 

one  side  of  the  rotation  point,  and  they  would,  therefore,  on  being 
lifted,  tilt  in  that  direction;  this,  however,  is  prevented  by  the 
cross-bar  resting  against  a  post  at  that  end  of  the  frame  toward 
which  the  inclination  tends.  The  scorifiers  are  clutched  by  the 
upper  prongs  of  the  tongs,  which  is  fastened  to  a  spring  on  a  post 
of  the  fork  below,  and  which  is  free  to  move  in  a  vertical  plane, 
the  pivotal  point  lying  over  the  spring  and  post.  By  bringing 


FIG.  21. — GAS-BURNING  MUFFLE-FURNACE. 

pressure  on  the  extended  ends  of  these  clutch  bars  behind  the 
pivot,  their  other  end  will  rise  above  the  scorifiers,  and  "thus 
release  them,  or  permit  the  placing  of  them  onto  the  tongs.  The 
pressure  exerted  on  the  rear  ends  of  the  clutches  is  accomplished 
by  means  of  a  cross-bar  fastened  to  a  spring  bar,  which  is  itself 
fastened  to  the  handle  of  the  instrument.  An  ordinary  mold 
with  20  holes,  arranged  to  receive  the  contents  of  the  scorifiers, 
goes  with  the  tongs. 


ASSAY    FURNACES    AND    TOOLS 


21 


Fig.  29  shows  a  device  to  charge  30  cupels  at  one  time.  It 
comprises  a  top  sliding  plate  with  openings  corresponding  ex- 
actly to  the  position  of  the  cupels.  The  openings  in  the  lower 
plate  correspond  with  those  of  the  upper  one;  the  plate,  how- 
ever, rests  on  two  adjacent  sides  extended  downward  at  right 
angles  to  the  plate  and  to  each  other,  thus  forming  two  closed 
sides  of  the  instrument;  one  at  the  front  and  the  other  at  the 


FIG.  23. — CRUCIBLE  TONOS. 


FIG.  24. — CUPEL  TONGS. 


FIG    25. — SCORIFIEB  TONGS. 


FIG.  26. — SCORIFIES  TONGS. 


right-hand  side.  The  height  of  these  sides  is  such  that  when  rest- 
ing on  the  bottom  of  the  muffle  the  bottom  plate  will  be  some  dis- 
tance above  the  cupels,  and  by  a  slight  pull  forward  and  a  push 
to  the  left  with  the  handle  of  the  instrument  the  set  of  cupels 
will  be  perfectly  alined  in  both  directions  and  the  apertures  in 
the  lower  plate  will  exactly  cover  the  tops  of  the  cupels.  The 
lead  buttons  are  placed  in  the  apertures  of  the  upper  plate 


22 


A    MANUAL    OF    FIRE    ASSAYING 


FIG.  27. — MULTIPLE  SCORIFIER  TONGS.     (Keller.) 


FIG.  28. — MULTIPLE  SCOHIFIEB  TONGS.     (Keller.) 


FIQ.  29. — CUPEL  CHARGING  DEVICE.     (Keller.) 


ASSAY   FURNACES   AND   TOOLS 


23 


and  rest  on  the  lower  plate  before  introducing  the  instrument  into 
the  furnace,  and  when  it  is  placed  over  the  cupels,  which  have 
been  properly  alined  in  the  muffle,  the  upper  plate  is  pushed 


FIG.  30. — POURINO  MOLDS. 


forward  to  a  stop-point,  bringing  the  apertures  of  the  two  plates 
into  register,  thus  causing  the  lead  buttons  to  drop  down  into 
the  cupels.  The  handle  of  the  upper  plate  runs  through  guides 


FIG.  31. — CUPEL  THAT. 


fixed  to  the  handle  of  the  lower  plate;  both  handles  are  connected 
with  a  spring,  which  acts  as  a  brake  when  the  upper  plate  is 
pushed  forward  to  drop  the  buttons,  and  also  serves  to  bring  it 


FIG.  32. — FIRE-CLAY  ANNEALING-CUP  TRAY. 


back  into  its  original  position,  in  which  the  buttons  cannot  drop 
through  the  apertures  in  the  lower  plate. 

Molds. — Fig.  30  shows  machined   cast-iron  molds   to  receive 


24  A   MANUAL    OF    FIRE   ASSAYING 

the  molten  fusions.  The  sharp  cone-shaped  mold  is  preferable 
to  the  shallow  hemispherical  type,  as  the  lead  buttons  are  then 
sharp  and  well  defined  and  separate  easily  from  the  slag.  The 
mold  is  best  made  with  a  screw-handle,  so  as  to  be  easily  repaired 
in  case  of  breakage.  The  inner  surface  of  the  molds  should  be 
machined  smooth,  to  permit  the  ready  separation  of  slag  and 
lead  button  from  the  mold.  For'scorification  fusions,  smaller 
molds  are  often  used. 


FIG    33. — FIRE-CLAY  CRUCIBLES. 

For  the  transfer  of  cupels  to  the  parting  room,  iron  cupel  trays, 
as  illustrated  in  Fig.  31,  are  used.  The  handle  is  removable,  and 
one  handle  serves  for  a  number  of  trays.  For  the  annealing  of 
gold  beads,  or  cornets,  fire-clay  trays  as  shown  in  Fig.  32  are 
employed.  Fire-clay,  however,  is  very  easily  broken,  and  more 
satisfactory  trays  are  made  of  sheet  iron  and  heavy  asbestos 
board. 

CRUCIBLES  AND  SCORIFIERS.— Fire-clay  crucibles  are  largely 
used  in  the  United  States,  and  fire-clay  ware  for  assay  purposes  is 
made  to  a  large  extent  in  some  of  the  western  States.  Following 
is  the  analysis  of  a  Colorado  crucible  clay:1 

1  Ed.  Orton,  "Assay  Crucibles.  Their  Clays."     Trans.  Am.  Ceramic  Soc.,  X,  (1908). 


ASSAY   FURNACES    AND    TOOLS 


25 


Loss  on  ignition 10.14  per  cent. 

Alumina 15.09  per  cent. 

Silica 71 .81  per  cent. 

Ferric  oxide 1 . 75  per  cent. 

Lime 0.14  per  cent. 

Magnesia 0.05  per  cent. 

Alkalies 1 .02  per  cent. 


FIG.  34. — SCOBIFIEBS. 

The  crucibles  are  rated  by  gram  capacity,  that  is,  by  the 
number  of  grams  of  ore  with  the  proper  amount  of  fluxes  neces- 
sary for  fusion  which  the  crucible  will  hold.  The  chief  sizes  are 
5,  10,  12,  15,  20,  30  and  40  grams;  of  these  the  20-  and  30-gram 
sizes  are  mostly  used,  the  20-gram  crucible  for  the  0.5  assay  ton, 
and  the  30-gram  for  the  1  assay  ton  fusions.  Fig.  33  shows  the 


FIG.  35. — BUCK  BOARD  AND  MULLEB. 

various  shapes  employed.     Imported  Hessian  triangular  crucibles 
and  sand  crucibles  are  also  used,  but  in  small  quantities. 

Imported  Battersea  clay  crucibles  give  good  satisfaction  and 
are  used  by  some  of  the  large  assay  offices  in  preference  to  domes- 
tic fire  clay  goods,  for  the  reason  that  their  quality  is  generally 
uniform  and  that  they  last  for  a  larger  number  of  fusions  than  the 


26 


A   MANUAL    OF    FIRE   ASSAYING 


poorer  grade  of  domestic  goods  which  are  sometimes  sold.  The 
highest  grade  of  domestic  material  is,  however,  in  most  cases  to 
be  preferred  as  being  fully  as  long  lived  and  cheaper. 

The  special  mixture  of  clays  and  their  treatment  for  crucible 
manufacture  is  generally  a  trade  secret,  jealously  guarded  and 
little  information  concerning  the  subject  is  available. 

Scorifiers  are  made  of  the  same  clays  as  the  crucibles  and  are 
designated  in  size  by  their  outside  diameters;  1.5-,  2-,  2.5-  and 
3.5-in.  sizes  are  made.  These  will  hold  a  volume  of  15  c.c., 


11  Mil 


FIG.  36. — BOCK  BOARD  BRUSHES. 

25  c.c.,  37  c.c.  and  100  c.c.,  respectively.  The  2.5-in.  scorifier  is 
the  one  commonly  used.  Fig.  34  shows  the  ordinary  type  of 
scorifiers.  Roasting  dishes  are  shallow  fire-clay  dishes  similar 
to  scorifiers,  but  not  so  thick.  They  are  rated  by  their  diameters; 
the  common  sizes  being  3,  4,  5  and  6  in.  Fig.  35  shows  the  or- 
dinary buck  board  and  muller,  and  Fig.  36  buck  board  brushes. 
For  the  description  of  other  minor  tools  and  apparatus,  as  screens, 
pliers,  and  crushing  and  grinding  machinery,  necessary  to  the 
assay  laboratory,  the  reader  is  referred  to  thd  voluminous  and 
well-illustrated  catalogues  of  the  assay  supply  houses.  Balances, 
weights,  sampling  tools,  cupels,  parting  devices,  etc.,  are  dis- 
cussed in  their  respective  chapters. 


CHAPTER  II 

DEFINITIONS;  REAGENTS;  THE  ASSAY  OF 
REAGENTS 

Assaying  includes  all  those  operations  of  analytical  chemistry 
which  have  for  their  object  the  determination  of  the  constituents 
of  ores  and  metallurgic  products.  Three  methods  are  used: 
(1)  Fire  assaying  (dry  methods);  (2)  gravimetric  analysis  (wet 
methods) ;  (3)  volumetric  and  colorimetric  analysis  (wet  methods) . 
This  work  treats  of  fire  assaying  only,  with  a  few  exceptions. 
The  quantitative  determination  of  the  following  metals  is  dis- 
cussed: gold,  silver,  platinum,  etc.,  lead,  antimony,  bismuth,  tin 
and  mercJury;  chiefly,  however,  gold  and  silver. 

Fire  assaying  comprises  the  separation  of  the  metal  sought 
from  the  other  components  of  the  ore,  by  heat  and  suitable 
fluxes,  and  then  the  weighing  of  it  in  a  state  of  greater  or  lesser 
purity. 

Gold  and  Silver. — Gold  and  silver  are  determined  in  their 
ores,  or  metallurgic  products,  by  collecting  them  with  lead, 
forming  an  alloy,  which  may  be  accomplished  either  by  the 
crucible  or  the  scorification  fusion,  the  lead  being  then  driven  off 
by  cupellation,  and  the  resultant  bead  of  the  gold  and  silver 
alloy  weighed.  The  separation  of  gold  from  silver  is  accom- 
plished by  parting  in  most  instances  with  nitric  acid,  rarely  by 
sulphuric  acid. 

In  order  to  successfully  collect  the  precious  metals  by  means 
of  lead,  it  is  essential  that  the  ore  be  mixed  with  suitable  fluxes, 
so  that  in  fusion  the  ore  is  thoroughly  decomposed  chemically, 
and  a  liquid  slag  of  the  proper  constitution  produced,  enabling 
the  lead  with  its  alloyed  gold  and  silver  to  settle  from  the  slag 
by  gravity,  thus  affording  a  ready  separation. 

27 


28  A    MANUAL    OF    FIRE    ASSAYING 

TABLE  I.— REAGENTS  COMMONLY  USED  IN  ASSAYING 


Name 


Formula 


Nature  (chemical) 


1.  Litharge  

PbO 

basic 

2.  Sodium  carbonate  

Na2C03 

basic 

3.  Sodium  bicarbonate  

NaHC03 

basic 

4.  Potassium  carbonate  

K2C03 

basic 

5    Silica 

SiO2 

acid 

6.  Borax  

Na2B4O7.10H,O 

acid 

7    Borax  glass 

Na2B4O7 

acid 

8    Fluorspar1 

CaF2 

neutral 

9.  Lime  

CaO 

basic 

10.  Hematite  

Fe203 

basic 

11.  Test    or    granulated   lead  \ 
Sheet  lead  / 

Pb 

basic 

12    Argol 

KHC4H4O6 

basic 

13    Charcoal 

c 

14    Coke  dust 

15    Flour 

16.  Lead  flux  

17.  Black  flux  

18.  Black  flux  substitute  

19.  Potassium  cyanide  

KCN 

neutral 

20    Potassium  nitrate 

KNO3 

basic 

21.  Salt  (sodium  chloride)  

NaCl 

neutral 

1.  Litharge  is  acted  on  in  the  crucible  by  reducing  agents, 
such  as  charcoal,  etc.,  and  metallic  lead  produced  as  follows: 


The  litharge  not  reduced  is  acted  on  by  silica  and  borax  glass, 
producing  silicates  and  borates  of  lead,  as  follows: 

PbO  +  SiO2=PbSiO3,  etc. 
Litharge  melts  at  884°  C.2 

.  2,  3.  Sodium  carbonate  is  decomposed  by  heat  in  the  cru- 
cible, as  follows,  at  high  temperature  not  usually  reached  in 
assaying: 

Na2C03  =  Na20  +  C02 

Or,  in  the  presence  of  silica,  at  lower  temperature, 
Na2CO3  +  SiO2  =  Na2SiO3  +  CO 

1  Not  decomposed  in  the  crucible  by  temperatures  ordinarily  used  in  assaying. 

a  Mostowitsch.  MetaUurgie,  IV,  648.     Doeltz  and  Mostowitsch.  Melallurgie,  IV,  290. 


THE   ASSAY    OF    REAGENTS  29 

The  Na2O,  with  silica,  forms  sodium  silicates,  as  Na2SiO3,  etc., 
which  are  very  fusible.  It  also  possesses  the  property  of  readily 
forming  sulphides  and  sulphates  and,  in  the  presence  of  metallic 
Fe,  of  freeing  lead  in  the  charge  from  sulphur. 

Na2CO3  melts  at  814°  C. 

Assayers  frequently  use  sodium  bicarbonate  in  place  of  cal- 
cined sodium  carbonate,  particularly  in  the  United  States,  on 
account  of  its  lower  cost.  Thus  while  refined  sodium  carbonate 
costs  8  cents  per  pound,  sodium  bicarbonate  costs  but  3  cents 
per  pound,  at  commercial  centers.  When  the  salts  are  calcu- 
lated to  the  basis  of  the  base  (Na20)  contained,  the  difference  in 
cost  is  not  so  wide,  still  the  bicarbonate  is  cheaper.  Neverthe- 
less it  is  preferable  to  use  the  carbonate,  since  the  great  amount 
of  gas  evolved  in  the  decomposition  of  the  bicarbonate  is  apt  to 
cause  mechanical  losses  in  the  assay.  Crude  sodium  carbonate 
or  soda-ash  may  be  used  costing  about  2  cents  per  pound.  In 
the  crucible  under  the  influence  of  heat  the  bicarbonate  decom- 
poses as  follows: 

2NaHC03  =  Na2C03  +  H2O  +  CO2 

4.  Potassium  carbonate  acts  in  a  similar  manner  to  sodium 
carbonate.     It  melts  at  885°  C. 

5.  Silica  is  a  powerful  acid  flux  and  combines  with  the  metallic 
oxides  or  bases  present  in  the  charge  to  form  the  slag,  which  is 
mainly  composed  of  silicates.     It  is  present  in  most  ores  in  con- 
siderable quantity,  ranging  from  small  amounts  in  basic  ores  to 
the  main  bulk  of  the  ore  in  quartz  ores.     It  melts  at  1775°  C.1 
(Quartz)  .—(Roberts- Austen,  1899.) 

6.  7.  Anhydrous    boric    acid    (B2O3),    Borax    (Na2O.2B203, 
10H20),  and  Borax  glass  (Na20.2B2O3)  or  anhydrous  sodium 
bi-borate.     Boric  acid2  readily  forms  borates  on  fusion  at  com- 
paratively high  temperature  with  lithium,  potassium,  sodium, 
and  silver  oxides,  generally  forming  orthoborates  (3Na2O.B2O3).' 
Boric  acid  does  not  readily  dissolve  silica,  but  sodium  or  potas- 
sium meta-borate  (Na2O.B203,  K2O.B2O3),  formed  probably  dur- 
ing the  fusion  of  borax  glass,  or  sodium  bi-borate  (Na2O.2B2O3), 
with  bases,  will  readily  dissolve  silica,  as  well  as  alumina  and 
chromic  oxide.    The  alkaline  meta-borates  are  markedly  volatile 
when  molten  and  deliquesce  in  the  air. 

1  Day  and  Shepard  give  the  melting-point  of  SiOz  at  approximately  1625°  C  ;  Jour.  Am. 
Chem.  Sac..  XXVIII,  1096. 

3  W.  Guertler,  SprecJtsaal,  XLV,  612;  Jour.  Soc.  Client.  Ind.,  Feb.  25,  1908,  158. 


30  A   MANUAL    OF    FIRE   ASSAYING 

Borates  are  classified  as  follows :  Ortho-borates,  e.g.  3CaO.B203; 
pyro-borates,  e.g.,  2CaO.B2O3;  sesqui-borates,  e.g.,  3Ca0.2B203; 
meta-borates,e.gr.,CaO.B2O3;and  bi-borates,  e.g.,CaO.2B203.  The 
following  borates  are  of  interest  to  the  assay er:  Magnesium 
ortho-borate,  3MgO.B203;  magnesium  pyro-borate,  2MgO.B2O3; 
the  corresponding  borates  of  nickel  and  cobalt;  the  ortho-,pyro-, 
meta-  and  bi-borates  of  calcium,  strontium  and  barium.  Lead 
oxide  forms  glasses  with  boric  acid  and  borax,  of  which  PbO.B2O3 
is  hard  like  flint  glass,  and  3PbO.  B2O3,  may  be  softened  in  boil- 
ing oil.  Other  substances  which  may  not  be  compounds  are: 
3ZnO.2B2O3;  3ZnO.B2O3;  MnO.B2O3;  3MnO.B2O3;  3MnO.2B2O3; 
CuO.B2O3;  3Cu2O.2B2O3;  and  3B2O3.2FeO.2Fe2O3.  Bismuth, 
antimony  and  arsenic  also  form  borates. 

Borax  and  borax  glass  are  fluxes  used  frequently  by  assayers. 
They  are  considered  acid  fluxes,  but  it  will  be  noted  from  the 
above  that  they  have  the  power  of  dissolving  silica  and  alumina 
and  will  hence  corrode  crucibles.  They  can  be  used  to  flux 
silica  to  a  certain  extent,  a  use,  however,  to  which  they  are  not 
put.  Sodium  bi-borate  has  the  property  of  passing  gradually 
from  the  liquid  to  the  solid  state  (amorphous)  and  vice  versa, 
under  ordinary  conditions  with  no  definite  freezing-  or  melting- 
point.  It  can  be  made  to  crystallize  or  freeze  at  a  definite  tem- 
perature only  under  the  influence  of  vibration  from  rapidly  re- 
peated shocks.  Crystallized  sodium  bi-borate  melts  at  742°  C.1 

The  use  of  borax  glass  as  a  flux  to  form  easily  fusible  borates 
with  metallic  bases  is  dependent  upon  the  liberation  of  boric  acid 
from  the  bi-borate,  in  the  presence  of  the  free  bases.  What  partic- 
ular borates  form  is  largely  a  question  of  temperature  attained. 
The  use  of  much  borax  gives  rise  to  hard  stony  slags,  very  tough, 
from  which  the  lead  button  separates  with  difficulty.  Often  a 
film  of  lead  will  adhere  to  the  slag,  causing  mechanical  loss. 
Slags  containing  much  borax  will  often  fly  to  pieces  suddenly, 
especially  when  touched  with  a  sharp  instrument,  while  cooling.2 
This  is  due  to  devitrification  of  amorphous  glassy  borates  and 
the  formation  of  definite  crystallized  borates. 

In  fluxing  ores  containing  zinc  it  is  to  be  noted  that  boric 
oxide,  either  alone  or  mixed  with  one-half  its  weight  of  borax, 
will  flux  zinc  oxide  into  a  very  fluid  slag,  which  is,  however,  very 
corrosive  to  clay  crucibles. 

1  Day  and  Allen,  Am.  Jour.  Sc.,  XIX,  102. 

E.  Clennell,  Eng.  and  Min.  Jour.,  LXXXVII,  696. 


THE   ASSAY    OF    REAGENTS  31 

8.  Fluorspar  is  occasionally  used  in  assaying.     It  melts  at  a 
comparatively  high  temperature,  1330°  C.,  but  when  fused  is  very 
thinly  fluid.     The  greater  part  of  it  remains  unchanged  through- 
out the  fusion,  and  hence  its  lime  cannot  be  considered  as  avail- 
able for  fluxing  silica.     It  gives  the  slags  containing  it  a  stony 
appearance.     Owing  to  its  great  fluidity,  it  has  the  property, 
shared  by  soda  and  litharge  to  some  extent,  of  holding  in  sus- 
pension unfused  particles,  thus  still  making  a  fluid  slag.     Where 
the  decomposition  of  the  ore  to  be  assayed  is  essential,  as  it  is 
in  most  cases,  its  use  is  not  to  be  advocated. 

9.  Lime  is  used  either  as  the  carbonate  or  as  the  oxide  or 
hydrate.     In  the  crucible  it  is  converted  into  oxide,  the  carbon- 
ate beginning  to  lose  its  C02  at  800°  C.     In  itself  it  is  extremely 
infusible  (1900°  C.;  Hempel,  1903),  but  with  silica,  when  joined 
with  other  bases  and  in  moderate  quantities,  it  makes  very  de- 
sirable slags.     It  is  found  in  many  ores.     Magnesia  acts  in  a 
similar  way.     Its  melting-point  is  2250°  C.     (Hempel,   1903.) 

10.  Hematite,  or  natural  ferric  oxide,  and  limonite,  are  of 
frequent  occurrence  in  ores,  and  are  sometimes  added  as  a  flux. 
Ferric  oxide  has  a  high  melting  point,  about  1560°  C.     In  the 
crucible  it  is  converted  by  reducing  agents,  such  as  argol,  char- 
coal, etc.,  to  ferrous  oxide  (FeO),  and  then  unites  with  silica  to 
form  silicates.     The  fact  that  it  is  reduced  to  ferrous  oxide,  con- 
versely gives  it  an  oxidizing  power.     Manganese  oxides  acting 
in  a  similar  way  are  also  frequently  found  in  ores.     Alumina, 
A1203,  is  often  found  in  ores,   and  unites  with  silica  to  form 
silicates.     It  has  no  oxidizing  power.     A12O3  melts  at  2010°  C, 
Kanolt  (1912.) 

11.  Test  lead  and  sheet  lead  are  used  chiefly  in  the  scorifica- 
tion  assay  and  in  cupellation.     In  both  of  these  operations  the 
lead  is  oxidized  by  the  oxygen  of  the  air  (2Pb  +  O2  =  2PbO)  to 
litharge.     In  the  scorification  assay  part  of  this  PbO  volatilizes; 
the  greater  part  becomes  fluid  and  holds  in  suspension  and 
solution  other  metallic  oxides  derived  from  ores,  thus  forming 
what  is  termed  an  oxide  slag.     In  cupellation,  part  of  the  lead 
is  volatilized  as  PbO,  and  part  is  absorbed  by  the  cupel  as  PbO. 
Lead  melts  at  326°  C. 

12.  Argol  is  a  crude  bitartrate  of  potassium,  separating  out 
in  wine  casks,  from  the  wine  on  standing.     On  heating,  it  breaks 
up  as  follows: 

=  K2O+5H20+6CO+2C 


32  A    MANUAL    OF    FIRE    ASSAYING 

The  carbon  and  carbon  monoxide  set  free  gives  it  its  reducing 
power.  The  K2O  left  acts  as  a  basic  flux. 

13,  14,  15.  Charcoal,  coke,  coal  dust,  sugar  and  flour  are 
reducing  agents  by  virtue  of  the  carbon  or  hydrogen,  or  both, 
that  they  contain. 

16.  Lead  flux  is  a  ready-prepared  flux  used  mainly  in  the 
assay  of  lead  ores  for  lead.     It  has  the  following  composition: 

Sodium  bicarbonate 16  parts 

Potassium  carbonate 16  parts 

Borax  glass 8  parts 

Flour 4  parts 

It  is  also  made  up  in  other  proportions. 

17.  Black  flux  is  made  of  1  part  KNO3  and  3  parts  argol, 
deflagrated.     It  is  sometimes  used  in  the  tin  and  lead  assay. 

18.  Black  flux  substitute  consists  of  3   parts  of  flour  and 
10  parts  of  NaHCO3.     It  is  used  in  the  tin  assay. 

19.  The   alkaline   cyanides   are   powerful   poisons   and   when 
powdering  them  for  use  as  a  flux  great  care  must  be  taken  not  to 
inhale  the  dust.     The  mortar  in  which  the  pulverizing  is  done 
should  be  covered  by  a  cloth  during  the  operation,  which  is  best 
conducted  at  an  open  window.     Two  kinds  of  commercial  cyanide 
may  be  readily  purchased  on  the  market.     1.  What  is  known  as 
"  potassium  cyanide,"  but  which  consists  of  the  mixed  cyanides 
of  sodium  and  potassium,  containing  varying  amounts  of  im- 
purities such  as  alkaline  carbonates,  sulphates,  etc.     The  quality 
is  expressed  by  the  cyanogen  content,  in  terms  of  KCN.     Thus 
"98  per  cent.  KCN"  is  in  common  use.     Without  going  into 
detail,  it  is  to  be  noted  that  salts  of  this  type  may  contain  con- 
siderable impurity,  although  rated  as  "98  per  cent.  KCN,"  and 
unless  known  to  be  good  should  not  be  used  in  the  tin  assay. 
Pure  potassium  cyanide,  c.p.  can  be  obtained  only  at  a  com- 
paratively high  price.     2.  Sodium  cyanide.     This  is  a  commercial 
salt  that  may  be   obtained  nearly  pure.     When  its  cyanogen 
contents  are  rated  at  125  to  130  per  cent.  KCN,  it  may  be  used 
with  safety  as  a  flux  for  the  tin  assay. 

A  sample  of  commercial  "98  per  cent.  KCN,"  impurities  not 
known,  had  a  freezing-point  of  526°  C.,  as  determined  in  the 
author's  laboratory. 

When  heated  somewhat  above  its  melting  point  in  the  presence 
of  air,  alkaline  cyanide  forms  cyanate  and  then  decomposes 
with  the  liberation  of  cyanogen.  Crucibles  in  which  it  is  used 


THE    ASSAY    OF    REAGENTS  33 

should  be  covered.     The  alkaline  cyanides  are  used  mainly  in 
the  assay  of  base  metals  as  bismuth,  lead,  tin  and  antimony. 

It  is  a  powerful  reducing  and  desulphurizing  agent,  acting  as 
follows: 

PbO  +  KCN  =  KCNO + Pb 
PbS  +  KCN  =  KCNS+Pb 

20.  Potassium  nitrate  or  niter  is  used  as  an  oxidizing  agent. 
With  metallic  lead  it  acts  as  follows: 

7Pb  +  6KNO3  =  7PbO  +  3K20  +  3N2  +  4O2  (approximately) . 

It  is  frequently  used  in  assaying  to  oxidize  impurities  in  the 
charge,  such  as  sulphur,  arsenic,  etc.  It  acts  as  a  basic  flux. 
Potassium  nitrate  fuses  at  339°  C. 

Sodium  nitrate  or  Chile  saltpeter  is  sometimes  used  in  place  of 
niter,  but  as  it  deliquesces  much  more  than  the  latter  it  is  not  so 
convenient. 

Other  oxidizing  agents  such  as  potassium  permanganate, 
potassium  ferri  cyanide,  etc.,  may  be  used  in  the  assay  of  impure 
ores,  but  are  more  expensive  and  not  any  better.  It  is  desirable 
to  dry  niter  at  100°  C.  before  use  and  then  keep  it  in  a  closely 
stoppered  bottle,  otherwise  it  will  be  weakened  per  unit  weight 
on  account  of  the  absorbed  moisture. 

21.  Salt  (NaCl)  is  used  as  a  cover.     It  is  very  thinly  fluid 
and  is  not  decomposed  during  the  fusion.     It  freezes  at  801°  C.1 

THE  ASSAY  OF  REAGENTS.  -  It  is  essential  for  the  assayer  to 
be  assured  of  the  fact  that  his  reagents  are  pure,  or  at  least  to  know 
to  what  extent  they  are  impure  and  what  the  impurity  consists 
of.  For  this  reason  it  is  necessary  to  examine  lots  of  reagents 
from  time  to  time,  as  they  come  into  the  laboratory,  by  approved 
chemical  methods,  tp  determine  their  purity.  Sometimes  re- 
agents or  fluxes,  as  a  result  of  being  left  exposed  in  the  laboratory, 
become  accidentally  or  purposely  "  salted  "  or  contaminated  with 
gold,  silver  or  base-metal  values.  A  blank  assay  for  metals  on 
the  reagents  will  readily  determine  this.  In  general,  it  may  be 
stated  that  the  labeling  of  a  chemical  "  c.  p."  does  not  necessarily 
make  it  so.  Borax  has  been  found  to  contain  platinum.2 

It  is  necessary  to  determine  the  silver  in  litharge  and  test 
lead,  as  these  two  reagents  frequently  contain  some  silver,  due 
to  their  being  usually  made  from  lead  bullion  refined  by  the 
Parkes'  or  zinc-desilverization  process,  which  leaves  some  silver 

1  W.  P.  White,  Am.  Jour.  Sci.,  XXVIII,  470. 

2  J  G.  Rose,  Jour.  Chem.  Met.  and  Min.  Soc.  S.  A.,  IX,  168. 


34  A   MANUAL    OF    FIRE   ASSAYING 

in  them.  As  litharge  is  almost  invariably  used  in  the  crucible 
assay,  and  test  lead  in  the  scorification  assay,  any  silver  or, 
possibly,  gold  introduced  into  the  results  by  their  use  must  be 
subtracted,  so  as  not  to  be  ascribed  to  the  ores.  Most  assay 
supply  houses  now  furnish  practically  silver-free  litharge  and 
lead  containing  only  traces' of  silver  and  no  gold. 

The-  method  of  determining  silver  and  gold  in  litharge  and 
test  lead  is  as  follows: 

The  following  charge  is  weighed  out  in  duplicate: 

Litharge -.  .  .       3  assay  tons 

Sodium  carbonate : 20  grams 

Silica 7  grams 

Argol 2  grams 

Borax  glass 5  grams  (as  a  cover) 

The  various  ingredients  are  put  from  the  scale  pan  on  a  sheet 
of  glazed  paper  and  thoroughly  incorporated  by  mixing.  It  is 
essential  to  weigh  the  litharge  and  argol  as  accurately  as  possible 
with  the  pulp  balances  in  use. 

The  incorporated  charge  is  then  transferred  to  a  20-gram 
crucible,  a  shallow  cover  of  borax  glass  being  put  on  top  of  the 
charge,  and  then  fused  in  the  muffle-furnace  for  from  25  to  35 
minutes  at  a  yellow  heat  (1000°  C.).  The  fusion  is  considered 
complete  when  the  charge  is  in  quiet  fusion,  that  is,  when  there 
is  no  more  bubbling  and  boiling  in  the  charge  and  when  the 
only  motion  observable  is  that  due  to  convection  currents.  The 
charge  is  then  poured  into  an  iron  mold  and  allowed  to  solidify, 
which  takes  approximately  10  minutes.  The  lead  button  is  then 
separated  from  the  slag  by  the  hammer  and  formed  into  a  cube. 
It  is  weighed  and  its  weight  recorded  in  grams  and  tenths  of  a 
gram  in  the  assay  note-book,  a  definite  assay  number  being 
assigned  to  this  assay  and  its  duplicate.  The  lead  button  is  then 
cupeled,  the  cupel  being  first  placed  in  the  muffle  for  10  to  12 
minutes  before  the  lead  button  is  dropped  into  it.  If  the  button 
weighs  from  15  to  20  grams,  as  it  should,  it  will  take  25  or  30 
minutes  to  finish  the  cupellation,  that  is,  to  drive  off  the  lead. 
The  end  of  this  operation,  in  this  particular  instance,  is  denoted 
by  the  darkening  of  the  small  silver  bead.  The  bead  is  then  re- 
moved from  the  cupel  after  this  has  become  cold,  flattened  on 
a  small  anvil  with  a  blowpipe  hammer,  cleaned  of  adhering 
bone-ash  from  the  cupel  by  a  button  brush,  and  weighed  care- 
fully on  the  assay  balances,  the  weight  being  recorded  in  milli- 


THE   ASSAY    OF    REAGENTS  35 

grams  and  hundredths  of  a  milligram.  The  weight  of  the  bead, 
divided  by  the  number  of  assay  tons  (3)  taken  in  the  assay,  gives 
the  number  of  ounces  contained  in  a  ton  (2000  Ib.)  of  litharge,  or 
the  number  of  milligrams  per  assay  ton  of  litharge.1  If  the 
presence  of  gold  is  suspected  in  the  litharge,  the  silver  bead  from 
the  cupellation,  after  weighing,  is  dropped  into  a  parting-cup 
filled  with  hot  nitric  acid  (9  parts  water  to  1  part  concentrated 
nitric  acid,  sp.  gr.  1.42),  which  will  dissolve  the  silver  and  leave 
the  gold  as  a  black  residue.  This  residue  is  washed  three  times 
by  decantation  with  cold  distilled  water,  carefully  dried  and 
annealed  at  a  red  heat  in' the  muffle;  after  cooling  it  is  weighed 
as  already  described  for  silver.  The  weight  of  the  gold  is  recorded 
and  then  subtracted  from  the  weight  of  the  original  gold  and 
silver  bead.  The  difference  in  weight  gives  the  amount  of  silver. 

To  determine  the  silver  and  gold  in  test  lead,  weigh  out  3 
assay  tons,  place  in  a  2.5-in.  scorifier,  add  a  pinch  of  borax  glass, 
and  scorify  in  the  muffle  at  a  yellow  heat  (1000°  C.).  As  the 
lead  oxidizes  to  litharge,  this  melts  and  forms  a  slag  which, 
owing  to  the  convexity  of  the  meniscus  of  molten  lead,  falls  to 
the  side  of  the  surface  and  forms  the  slag  ring,  leaving  a  disk  of 
fresh  lead  exposed.  The  scorification  is  finished  when  the  slag 
finally  covers  all  the  lead.  The  charge  is  then  poured  into  an 
iron  mold,  the  further  method  of  procedure  followed  being  iden- 
tical with  the  one  described  for  the  litharge  assay. 

It  is  possible  to  obtain  test  and  sheet  lead  with  only  traces 
of  silver,  and  litharge  practically  free  from  silver.  It  is  often 
desirable  that  the  litharge  should  contain  a  uniform  amount  of 
silver,  for  whenever  low-grade  gold  ores,  deficient  in  silver,  are 
assayed,  silver  will  have  to  be  added  at  some  stage  of  the  assay 
in  order  to  insure  parting,  or  the  complete  separation  of  the  gold 
from  the  silver.  In  assaying  very  low-grade  gold  ores,  in  which 
practically  only  gold  is  present,  the  final  bead  might  be  so  small 
as  to  sink  into  minute  cracks  in  the  cupel  and  thus  be  lost.  The 
addition  of  silver  in  this  case,  either  by  adding  it  in  the  metallic 
state  or  by  its  presence  in  the  litharge,  obviates  this  difficulty. 

Litharge  will  frequently  contain  from  0.20  to  0.32  mg.  of 
silver  per  assay  ton.  It  is,  however,  not  safe  to  assume  the 
above  figures.  The  test  lead  ordinarily  bought  from  the  supply 
houses  contains  only  traces  of  silver. 

1  For  a  discussion  of  weights  used  in  assaying,  cupellation  and  weighing,  reference  should 
be  made  to  these  subjects- 


CHAPTER  in 
SAMPLING 

Proper  sampling  is  of  the  utmost  importance,  for  unless  the 
sample  to  be  assayed  accurately  represents  the  lot  of  ore  or 
metallurgic  product  from  which  it  is  taken,  in  other  words,  unless 
it  is  a  true  sample,  the  greatest  care  in  the  assay  itself  means 
nothing.  Large  amounts  of  money  are  involved  in  settlements 
made  on  the  assay  of  final  samples  representing  many  tons  of 
rich  ore,  matte,  bullions,  etc.  Mills  and  smelters  purchase  ores 
by  the  carload  on  the  assay  of  the  final  sample,  and  even  slight 
errors  mean  loss  either  to  the  shipper  or  the  purchaser.  Where 
so-called  "specimen"  assays  are  made,  the  sampling  of  the 
small  amount  of  pulp  is  usually  a  simple  matter,  although  accu- 
racy is  also  required.  In  most  cases  the  samples,  representing 
large  lots,  are  handed  to  the  assayer,  so  that  he  is  usually  not 
directly  concerned  as  to  how  the  samples  were  obtained;  but  in 
general  he  should  be  familiar  as  to  how  sampling  is  conducted. 
Sampling  may  be  classified  under  two  heads: 

1.  Hand  sampling: 

a.  Coning  and  quartering. 

b.  Alternate  shovels. 

c.  Split  shovels. 

d.  Riffling. 

2.  Machine  sampling: 

a.  Part  of  the  ore  stream  for  the  whole  time. 
6.  The  whole  of  the  ore  stream  part  of  the  time. 

Whatever  the  method  of  sampling  used,  a  distinct  relation 
must  exist  between  the  weight  of  the  sample  and  the  size  of  the 
ore  particles.  Thus,  if  the  ore  particles  are  large  (10  to  12  in. 
diameter)  a  large  sample  must  be  taken;  if  the  particles  are 
small  (0.10  to  0.20  in.)  a  small  sample  will,  if  properly  taken, 
accurately  represent  the  lot  of  ore.1  An  old  rule  in  force  on 

1  Brunton,  "The  Theory  and  Practice  of  Ore  Sampling,"  Trans  A.  I.  M.  E.,  XXV,  826; 
and  Trans.,  A.  I.  M.  E.,  XL,  567.  Notes  on  Sampling,  Min.  Reporter,  XLV,  Nos.  7-16 
(inclusive). 

36 


SAMPLING  37 

Gilpin  County,  Colorado,  ores,  carrying  from  1  to  4  oz.  gold, 
illustrates  this: 

Diam.  of  largest  piece,  in  inches 0.04         0.08     0.16     0.32     0.64     1.25       2.50 

Minimum  weight  of  sample,  in  Ibs 0.0625     0.50     4  32     256       2048     16348 

The  proper  weight  of  sample  for  any  desired  size  of  ore  particle 
is  obtained  by  multiplying  the  known  weight  for  the  given  size 
by  the  cube  of  the  ratio  of  the  desired  size  to  that  of  the  given 
size. 

As  an  example  of  mill  practice  by  machine  sampling  on 
Cripple  Creek  ores  of  from  2  to  6  oz.  gold  per  ton,'  the  following 
is  given: 

The  ore  is  crushed  to  pass  a  1.5-in.  ring,  and  from  the  total 
bulk  a  Vezin  sampler  cuts  out  one-fourth.  This  is  passed  to 
crushing  rolls,  which  reduce  it  to  0.25-in.  size.  It  is  then  ele- 
vated to  another  Vezin  sampler,  which  takes  out  one-tenth  of  the 
bulk,  the  final  sample  being  one-fortieth  of  the  ore,  or  2.5  per  cent. 
This  is  then  cut  down  and  crushed  finer  and  sampled  in  the 
usual  way  (alternate  shovels,  etc.),  described  further  on.  In 
smelting  works,  where  it  is  desirable  to  have  the  product  going 
to  the  furnaces  as  coarse  as  possible,  the  above  method  is  modi- 
fied by  not  crushing  so  fine  and  by  taking  larger  samples;  or  hand 
sampling  is  employed.  The  size  of  the  sample  depends  not  only 
on  the  size  of  the  ore  particles,  but  also  on  the  nature  of  the 
ore.  If  the  values  are  uniformly  distributed,  smaller  samples 
will  do  than  are  necessary  where  they  are  "spotted"  or  irregu- 
larly distributed.1  While  machine  sampling,  with  properly  con- 
structed apparatus,  is  largely  in  use,  and  is  most  desirable  when 
applicable,  hand  sampling  may  be  accurately  performed;  it  is 
still  widely  used  by  smelting  plants,  as  it  avoids  crushing  a  large 
part  of  the  ore.2 

Coning  and  Quartering. — The  method  of  "coning  and  quarter- 
^ing"  has  been  in  use  for  many  years,  and  is  still  employed,  but 
it  is  being  displaced  largely  by  the  "  alternate-shovel."  method. 
Coning  and  quartering,  unless  carefully  performed,  which  is  diffi- 
cult to  do,  is  apt  to  be  inaccurate.  In  this  method,  the  thorough 
mixing  of  the  ore  is  essential,  and  the  mixing  is  supposed  to 
be  effected  by  coning.  The  cone  is  built  up  by  men  moving 
around  the  circumference  of  a  circle  and  shoveling  the  ore  upon 

1  L.  T.  Wright,  Element  of    Chance  in  the  Sampling  of  Ores,  Min.  Mag.,  Ill,  353 
(1910). 

2  For  a  good   discussion  of    Machine  Sampling,   consult  A.  W.  Warwick,  "Notes  on 
Sampling,"  published  by  the  Industrial  Pub.  Co.,  Denver,  Colo. 


38  A    MANUAL    OF    FIRE    ASSAYING 

the  point  of  a  cone  formed  by  the  angle  of  repose  of  the  material 
falling  vertically  upon  one  point.  The  samplers — from  4-  to  8 
men — move  so  as  to  be  always  diametrically  opposite  each  other. 

In  order  to  fix  the  point  of  the  cone,  a  rod  is  driven  into  the 
ground  as  a  guide.  It  is  evident  that  the  shoveling  must  be 
very  conscientiously  done  in  order  to  have  the  ore  distribute 
itself  uniformly  (fine  and  coarse)  over  the  surface  of  the  cone; 
but  this  uniformity  is  essential  to  the  obtaining  of  a  true  sample. 
When  the  cone  has  been  built  up,  it  is  then  pulled  down  by  the 
men  walking  around  the  pile  and  scraping  the  ore  from  the  apex 
to  the  base,  until  a  flat  plaque  of  ore  is  made  about  12  or  18  in. 
thick.  Then,  in  the  form  of  a  cross,  plates  of  iron  are  carefully 
centered  on  the  pile  and  driven  in,  dividing  the  plaque  into 
quarters.  Two  opposite  quarters  are  removed  to  the  bins,  and 
the  other  two,  representing  the  sample,  are  reshoveled  into  a 
cone  and  the  operation  repeated.  The  ore  is  then  recrushed  and 
coned  and  quartered  again,  until  finally  a  sample  of  from  25  to 
30  Ib.  is  obtained.  The  number  of  recrushings  depends  upon 
the  size  of  the  first  sample  and  the  nature  of  the  ore.  The  sample 
is  then  ground  fine  and  prepared  for  the  assay  office  by  cutting 
down  with  a  split  sampler  or  other  approved  device.  The  whole 
process  is  slow  and  laborious.  Three  men  can  handle  from  20  to 
25  tons  of  sample  per  shift  at  a  cost  of  from  45  to  50  cents  per  ton. 

The  Alternate-shovel  Method. — The  fundamental  law  of 
sampling  may  be  stated  thus:  In  order  to  properly  take  a  sample 
of  ore,  it  is  necessary  to  take  the  sample  frequently,  or  in  as 
many  places  as  possible,  and  to  take  the  same  quantity  each 
time  at  regular  intervals.  These  conditions  are  fulfilled  by  the 
"alternate-shovel"  method,  which  is  conducted  as  follows: 

The  ore  from  the  cars  is  dumped  on  a  platform  and  men  with 
the  proper  sized  and  shaped  shovels  put  it  into  the  bins,  taking 
out  for  the  sample  a  certain  number,  dependent  on  the  nature 
and  size  of  the  ore  pieces;  e.g.,  nine  shovels  are  thrown  into  the 
bins  and  every  tenth  shovel  is  taken  as  a  sample.  If  the  ore  is 
difficult  to  sample,  sample  shovels  may  be  taken  more  frequently; 
or  if  the  ore  is  uniform,  less  frequently.  It  is  usual  to  cut  out 
from  one-fifth  to  one-twentieth  of  the  ore.  The  alternate-shovel 
method  possesses  the  following  advantages: 

1.  It  is  more  reliable  and  accurate  than  coning  and  quartering. 

2/  It  is  cheaper  in  operation. 

3.  It  is  quicker. 


SAMPLING  39 

The  "quartering"  and  the  "split-shovel"  methods  are  not 
reliable  and  need  not  be  described. 

At  the  plant  of  the  Standard  Smelting  Company,  at  Rapid 
City,  S.  Dak.,  the  shovel  sample  is  passed  to  a  Blake  crusher 
with  a  9  X 15  in.  mouth  opening,  having  an  A  discharge,  so  as  to 
halve  the  crushed  sample.  One  of  the  halves  is  fed  directly  to 
a  pair  of  24X12  in.  rolls,  the  discharge  from  which  is  again 
automatically  halved.  If  a  100-ton  lot  is  taken  as  a  unit,  the 
sample  at  this  point  is  2.5  tons  (taking  every  tenth  shovel),  with 
no  particle  larger  than  0.375  in.  in  diameter.  The  rolls  discharge 
directly  upon  a  plate-iron  floor,  where  the  ore  is  reshoveled, 
every  fifth  or  tenth  shovel  being  taken  as  a  sample,  which  now 
amounts  to  1000  or  500  Ib.  This  is  put  through  a  pair  of  12  X 
12  in.  sampling  rolls  and  crushed  fine,  and  then  sampled  by  a 
large  Jones  split  or  riffle  sampler,  which  takes  halves,  until  finally 
a  sample  of  between  15  and  20  Ib.  is  arrived  at.  This  is  put 
through  a  small  cone  grinding  mill,  and  after  a  determination  of 
moisture  on  the  sample  floor  is  sent  to  the  assay  office.  Here 
it  is  cut  down  to  about  2  Ib.  by  a  small  Jones  sampler,  and  then 
crushed  on  a  buck  board  to  pass  a  120-mesh  screen,  furnishing 
the  assay  sample.  This  sample  is  supposed  to  contain  no  mois- 
ture, as  this  was  eliminated  on  the  sample  floor,  where  the  per- 
centage of  moisture  is  determined;  but  as  all  settlements  are 
made  on  dry  samples,  the  final  assay  sample  is  again  heated  at 
100°  C.  for  some  time  in  order  to  expel  any  moisture  which  the 
sample  may  have  absorbed  in  its  passage  from  the  sampling 
works  to  the  assay  office.1  The  assay  sample  is  divided -into 
4  parts  and  put  in  paper  sacks.  One  part  is  assayed  by  the 
seller  of  the  ore  or  product;  one  part  by  the  purchaser;  a  third 
part  is  kept  for  emergency;  and  a  fourth  part  is  laid  aside  for  an 
umpire  assay,  if  such  becomes  necessary. 

The  assays  made  by  the  seller  of  the  ore  and  those  made  by 
the  purchaser  of  the  ore  are  called  control  assays.  If  the 
seller  and  purchaser  agree  within  a  certain  limit,  depending  on 
the  value  of  the  ore,  settlement  is  made  on  the  purchaser's  assay, 
or  sometimes  on  the  average  of  the  two  assays.  If  they  do  not 
agree,  it  is  the  practice  for  the  buyer  and  seller  to  reassay  their 
own  samples  or  to  exchange  pulp  samples  and  reassay.  If  they 
do  not  then  agree,  an  umpire  assayer  is  chosen  who  makes  an 

1  G.  A.  James,  Eng.  and  Min.Jour.,  XC,  1047,  "Moisture  as  a  Source  of  Error  in  Assay 
Reports." 


40 


A    MANUAL    OF    FIRE    ASSAYING 


umpire  assay,  by  the  results  of  which  all  parties  abide,  and  on 
which  settlement  is  made.  The  party  that  is  farthest  away 
from  the  result  of  the  umpire  has  to  pay  for  the  assay. 

Controls  are  made  with  three  check  assays,  and  umpires  with 
four  check  assays.  In  sampling  small  lots  in  the  laboratory 
and  cutting  down  for  the  assay  sample,  the  principles  already 
enumerated  also  apply.  Riffle  samplers  are  commonly  used 
as  well  as  the  coning  and  quartering  method,  although  this  last 
is  not  recommended,  even  for  small  lots.  The  final  pulp  sample 
is  put  through  a  100-  or  120-mesh  screen;  for  high-grade  material, 
150-  to  200-mesh  is  better.  It  is  then  thoroughly  mixed  on  a 
rubber  sheet  or  on  heavy  glazed  paper,  spread  out  in  a  thin, 


FIG.  37. — JONES  Ri 


broad  plaque  0.25  in.  thick,  and  small  lots  taken  with  a  spatula 
at  regular  intervals,  until  the  required  weight  is  obtained.  Fig. 
37  shows  the  Jones  riffle  sampler  and  Fig.  38  the  Umpire  mechan- 
ical ore  sampler. 

The  form  with  pans  is  preferable  to  that  which  has  half  of 
its  riffles  closed.1  The  samplers  should  have  an  even  number 
of  riffles,  and  not  less  than  12  of  them  for  best  work.  The 
sampling  shovels  should  be  of  the  width  of  the  sampler  and  per- 
fectly straight  across  the  edge  so  that  the  ore  will  fall  uniformly 
into  the  riffles,  and  in  equal  quantities  into  each  riffle.  Large 
riffle  samplers  for  400  to  500  Ib.  samples  are  used  in  sampling 
works.  The  riffle  sampler  when  properly  used  is  an  apparatus 
that  will  do  accurate  work. 

Great  care  should  be  taken  to  clean  all  sampling  apparatus  after 
sampling  each  lot,  so  as  to  avoid  "salting"  samples.  This  also 
applies  to  all  the  crushing  machinery  employed  in  the  sampling. 

1  L.  D.  Huntoon,  "  Accuracy  of  Mechanical  and  Riffle  Ore  Samplers,"  Eng.  and 
Min.  Jour.,  XC,  62. 


SAMPLING 


41 


Sampling  Lead  Bullion.1 — Lead  bullion  is  molded  into  bars 
of  approximately  80  Ib.  weight  and  shipped  in  this  form.  The 
best  method  of  sampling  is  to  take  dip  samples  at  regular  intervals 
while  a  lot  of  bars  are  being  molded  at  the  furnace.  When  the 
solid  bars  are  to  be  sampled,  about  the  only  reasonably  accurate 


FIG.  38. — UMPIKE  ORE  SAMPLER. 

method  is  to  take  a  "  saw  sample."     This  is  carried  out  as  follows : 
Out  of  a  lot  of  bars,  every  fifth  or  tenth  bar  is  sawed  across  the 
middle  into  two  pieces.     The  saw  dust  is  then  further  cut  down 
to  the  proper  amount  for  sample  and  assayed. 
Copper  bullion  is  sampled  in  a  similar  manner.2 
Chip  or  gouge  samples  are  almost  invariably  inaccurate. 

1  G.  M.  Roberts  "Experiments  in  the  Sampling  of  Silver  Lead  Bullion,"  in  Trans. 
A.  I.  M.  E.t  XXVIII,  413.  Edward  Keller,  "The  Distribution  of  the  Precious  Metals 
and  Impurities  in  Copper,"  ibid.,  XXVII,  106. 

2Wm.  Wraith,  "Sampling  Anode  Copper,  with  Special  Reference  to  Silver  Content," 
Trans.  A.  I.  M.  E.  Bui.,  39,  209  (1910).  D.  M.  Liddell,  "Sampling  Copper  Bars,"  Eng. 
and  Min.  Jour.,  XC,  752,  897,  953,  1095.  "Saw  Sampler  for  Copper  Bars,"  Eng.  and  Min. 
Jour.,  XC.,  640. 


CHAPTER  IV 
WEIGHING;  BALANCES  AND  WEIGHTS 

BALANCES. — The  balance  used  in  weighing  the  minute  quan- 
tities of  gold  and  silver  is  a  delicate  piece  of  apparatus  and  must 
be  carefully  adjusted  and  handled  in  order  to  give  accurate  re- 
sults. The  balance  should  be  set  upon  a  firm  foundation,  not 
subject  to  vibration;  otherwise  it  is  apt  to  be  frequently  thrown 
out  of  adjustment.  Stone  or  concrete  piers  set  some  distance 
into  the  ground  and  free  from  the  floor  are  the  best  foundations, 
when  the  vibrations  induced  by  moving  machinery  are  absent. 
Where  such  vibrations  occur,  insulated  shelf  supports  should 
be  used. 

Construction. — The  balance-beam  is  made  of  aluminium,  gold- 
plated  brass,  special  silver  aluminium  alloys,  etc.,  and  as  light  as 
possible  consistent  with  the  requisite  strength.  The  material 
from  which  it  is  made  should  be  non-magnetic,  and  have  a  small 
coefficient  of  expansion,  so  that  temperature  changes  will  have 
but  slight  effect  on  the  length  of  the  beam.  The  pan-hangers 
are  frequently  of  a  nickel-silver  alloy,  or  of  german  silver,  and 
the  pans  of  aluminium.  The  standards  and  other  metal-work  are 
best  made  of  gold-plated  brass.  The  knife-edges  and  the  plates 
on  which  they  rest  are  made  of  agate,  accurately  polished  and 
ground  true.  The  balance-beam  has  three  knife-edges,  which 
should  be  in  line  in  the  same  plane  in  order  to  give  equal  sen- 
sibility with  varying  loads.1  The  two  balance-arms,  or  the 
distance  from  the  central  knife-edge  to  each  of  the  outer  knife- 
edges,  should  be  equal  in  length.  This  can  never  be  absolutely 
accomplished,  but  may  be  very  closely  approximated.  The 
accompanying  illustration  (Fig.  39)  shows  the  essential  features 
of  the  balance. 

When  the  small  weight  ra'  is  put  into  the  pan  it  will  cause  a 
deflection  of  the  pointer,  and  the  center  of  gravity  of  the  balance 
system  shifts.  The  condition  of  equilibrium  is  then  expressed 
by  the  equation 

Mx  =  m'x' 

1  Gottschalk,  "The  Balance,"  in  West.  Chem.  and  Met.,  II,  April,  May  and  June,  1906. 
42 


BALANCES    AND    WEIGHTS 


43 


£>.... 
C,  C'. 
c.  g... 

Y. ... 
x. . . . 


FIG.  39. — DIAGRAM  OF  ASSAY  BALANCE 


.  the  central  knife-edge. 

.  the  outer  knife-edges. 

.adjustment  for  center  of  gravity  of  the  balance  system. 

.adjustments  for  equal  moment  of  arms. 

.  center  of  gravity  of  the  balance  system. 

.  pointer-arm. 

.  distance  of  deflection  of  center  of  gravity,  or  the  gravity  lever-arm. 

.  lever-arm  of  small  weight  TO'  in  pan. 

.small  weight. 

.  mass  of  balance  system. 


44  A    MANUAL    OF    FIRE    ASSAYING 

From  this  it  follows  that  if  M  increases,  that  is,  if  the  mass 
of  the  balance  system  becomes  greater,  and  other  conditions 
remain  constant,  the  weight  m'  must  increase  to  cause  the  same 
deflection;  i.e.,  the  sensibility  of  the  balance  will  be  lessened,  the 
sensibility  being  the  amount  of  deflection  caused  by  a  given 
mass.  Assay  balances  must  show  a  sensibility  of  at  least  one- 
half  division  of  the  scale  with  a  weight  of  0.01  mg.  or  even  0.005 
mg.  This  shows  the  necessity  for  an  extremely  light  construc- 
tion, or  a  small  mass  of  the  balance  system.  It  is  evident  from 
the  equation  that  the  sensibility  might  be  preserved  by  increas- 
ing xf  i.e.,  by  lengthening  the  arms  of  the  balance;  in  practice, 
however,  this  would  also  very  materially  increase  M,  so  that  the 
gain  is  more  apparent  than  real.  Long  arm  balances  are  also 
very  slow  of  vibration.  Formerly,  long  arm  balances  were 
common,  but  in  modern  assay  balances  the  arm  rarely  exceeds 
2.5  in. 

From  the  equation  it  also  follows  that,  if  the  center  of  gravity 
of  the  balance  system  is  placed  at  A,  the  knife-edge,  x  becomes 
zero  and  we  have 


M  Xo  =  m'x';  or  m'xf  =  o; 

or  m'  approaches  o,  for  x'  is  practically  constant;  in  other  words, 
the  balance  will  become  extremely  sensitive,  an  infinitely  small 
weight  in  the  pan  causing  rotation.  While  the  balance  would 
be  very  sensitive,  it  would  also  be  very  unstable  and 
"  cranky." 

In  designing  balances  it  is  hence  very  important  to  preserve  a 
mean  between  high  sensibility  and  stability,  and  as  this  latter  is 
obtained  mainly  by  lowering  the  center  of  gravity,  which  lessens 
sensibility,  in  part  by  increasing  friction  on  the  knife  edges, 
these  must  be  constructed  with  extreme  care,  and  must  be  as 
nearly  true  as  possible.  Most  assay  balances  are  provided  with 
fall  away  pan  rests  operated  by  a  thumb  screw  on  the  outside 
of  the  case.  When  this  screw  is  turned  to  the  left,  the  rests  drop 
away  from  the  pan,  and  on  further  turning  the  balance  beam  is 
lowered  and  set  free  so  that  the  central  knife  edge  rests  on  its 
bearing,  and  the  balance  is  free  to  act. 

The  screw-ball  D  is  provided  to  adjust  the  center  of  gravity, 
which  should  be  somewhat  below  the  knife-edge  A.  The  center 
of  gravity  is  adjusted  so  that  a  weight  of  0.01  mg.  in  the  pan  or 
on  the  beam  will  cause  a  deflection  of  from  one-half  to  one  divi- 


BALANCES   AND    WEIGHTS  45 

sion  of  the  pointer.  The  lower  the  center  of  gravity  of  the  balance 
system,  the  more  rapid  the  oscillation  of  the  balance.  The 
higher  or  the  nearer  the  point  of  suspension,  the  slower  the  oscil- 
lations and  the  greater  the  sensibility. 

Weighing. — Before  weighing,  the  balance  is  always  thoroughly 
cleaned  in  every  part  from  dust  by  a  soft  camel's-hair  brush, 
made  perfectly  level  by  adjusting  the  leveling  screws,  and  the 
pointer  standardized  to  o  by  the  little  thumb-screws  C,  C'. 
To  do  this,  the  balance  is  set  in  motion  until  the  pointer  swings 
to  from  5  to  8  divisions  on  the  scale  each  side  of  the  zero 
mark.  If  the  balance-arms  are  equal  in  moment,  the  pointer 
will  swing  practically  an  equal  number  of  divisions  on  each  side, 
losing,  however,  a  trifle  on  each  swing,  thus:  +8,  — 7.75, +7.5, 
—  7.25, +7, —  6.75,  etc.,  the  loss  being  due  to  friction  and  to  a 
gradual  settling  back  into  equilibrium.  If  the  swings  are  not  as 
outlined,  the  adjustment  is  made  until  they  become  so.  The 
balance  is  then  tested  for  sensibility  as  described;  and  the  adj  ust- 
ment  made  for  it,  if  necessary,  by  moving  the  center  of  gravity. 
If  the  balance-arms  are  suspected  of  being  unequal  in  length 
(though  this  is  rare  in  good  balances),  weighing  by  "substi- 
tution," or  double-weighing  is  adopted.  In  this  method,  the 
object  to  be  weighed  is  placed  first  in  one  pan  and  weighed,  and 
then  in  the  other,  the  true  weight  being  the  square  root  of  the 
product  of  the  two  weights  found.  When  the  sensibility  of  the 
balance  is  accurately  known,  no  adjustment  for  equal  moment 
of  arms  need  be  made,  but  weighing  may  be  done  by  deflection, 
after  the  true  zero  or  equilibrium  point  is  found.  This  is  found 
as  follows:  Start  the  balance  swinging  and  count  swings  to  the 
left  as  minus  and  to  the  right  as  plus.  Suppose  the  swings  are 
as  follows:  —  8,  +3,  —  7.5.  The  zero-point  then  is 

—  +3  =  —  4.75  (divisions). 

=—2.375  (divisions). 

or  the  true  zero,  or  true  "point  of  rest"  is  2.375  divisions  to  the 
left  of  the  zero  mark  on  the  scale. 

Then  place  the  particle  to  be  weighed  on  the  right-hand  pan 
and  weigh  again  to  determine  the  point  of  rest  under  these  con- 
ditions. The  swings  are  as  follows:  —10, +2,  — 9.5.  The  sensi- 


46  A   MANUAL    OF    FIRE    ASSAYING 

bility  of  the  balance  being  0.5  division  deflection  for  each  0.01  mg. 
the  new  zero-point  is 

) 

+2  =-7.75  (divisions) 


-7.75 

—  -  —  =  —3.875  (divisions),  new  point  of  rest, 

and  the  weight  of  the  particle  is  the  difference  in  deflection 
between  the  two  points  of  rest,  (3.875  —  2.375  =  1.5  division) 
divided  by  0.5,  or  0.03  mg.  In  practice,  in  place  of  two  readings 
on  one  side  and  one  on  the  other  of  the  zero,  it  is  better  to  make 
three  and  two  readings  respectively. 

This  method,  however,  is  not  generally  to  be  recommended; 
the  "rider"  should  be  used  for  the  determination  of  the  fractional 
parts  of  the  milligram.  The  balance  should  also  be  adjusted  for 
equal  moment  of  arms,  as  described,  before  weighing. 

In  order  to  detect  inequality  in  the  length  of  the  arms,  stand- 
ardize the  balance  to  the  true  zero,  place  a  1-gram  weight  on  the 
right  pan,  and  an  old  or  worn  1-gram  weight  on  the  left  pan, 
and  bring  the  balance  into  approximate  equilibrium  by  adding 
minute  quantities  of  old  rider  wire  to  the  short  weight. 

Let  the  gram  weight  in  the  right  pan  be  called  A. 

Let  the  counterpoise  in  the  left  pan  be  called  B. 

Let  R  be  the  right  lever-arm  and  L  the  left  lever-arm. 

Determine  the  zero-point  of  the  balance  in  the  manner  de- 
scribed. If  this  zero-point  differs  from  that  of  the  unloaded 
balance,  bring  the  balance  to  the  old  zero-point  by  moving  the 
rider  on  the  left  or  right  arm,  as  required. 

Let  the  weight  indicated  by  the  rider  be  called  +m  or  —  m, 
as  it  may  act  with  or  against  B  to  bring  the  balance  system 
back  to  the  original  zero-point. 

Now  shift  the  weight  A  to  the  left  pan  and  B  to  the  right 
pan;  remove  the  rider  and  again  determine  the  zero-point,  and 
then  manipulate  the  rider  to  bring  the  balance  system  to  the 
zero-point  of  the  unloaded  balance  and  call  the  weight  indicated 
by  the  rider  ±n,  as  it  may  act  with  or  against  A.  The  following 
equations  will  then  result: 

1.  AR  =  (B±m)L  A  =  (B±m)L 

R 

2.  BR  =  (A±n}L 


BALANCES   AND    WEIGHTS  47 


L  L  A+B 

3.  A+B  =  (B  +  A±m±n)  --,  or 


R     (B  +  A±m±ri) 

L  ±m±n  L  ±m±n 

4.       =1 — ,  or,  approximately,  —  =  1  —  —     — . 

R  A+B±m±ri  R  2A 

If  m  =  -  n,  or  the  reversal  of  the  masses  shifts  the  zero- 
point  exactly  as  much  to  one  side  as  it  was  before  on  the  other 

L          L 

of  the  actual  o,  the  balance  has  equal  arms;  i.e.,  --  =  1,  —should 

R          R 

not  exceed  1±  0.000003. 

Some  assayers  weigh  by  "no  deflection."  They  adjust  the 
balance  to  the  true  zero,  place  the  bead  to  be  weighed  in  the 
right-hand  pan,  and  then  by  the  addition  of  weights  and  the 
moving  of  the  rider  by  repeated  trials,  balance  the  bead,  so  that 
finally,  when  the  balance  is  lowered  gently  on  its  knife-edge,  no 
deflection  of  the  pointer  takes  place.  This  method,  however,  is 
not  recommended,  as  it  disregards  friction  and  inertia,  and  for 
small  weights  gives  inaccurate  results. 

PRACTICAL  NOTES  ON  THE  ASSAY  BALANCE.  -  in  those  labor- 
atories where  the  balance  cannot  be  supported  on  stone  piers 
trouble  may  be  experienced  from  jarring  of  the  balance.  This 
can  largely  be  eliminated  by  supporting  the  levelling  screws  on 
truncated  pyramids  cut  out  of  rubber  packing,  making  the  lower 
base  of  the  support  2  in.  square  and  the  upper  one  1  in.  square, 
with  a  thickness  of  about  1  in.  A  small  square  of  ground  glass 
may  be  cemented  to  the  top  of  each  support  to  take  the  thrust  of 
the  levelling  screw.  Another  method  of  avoiding  the  jar  is  to 
bore  four  holes  \  in.  deep  into  the  balance  table  top,  and  insert 
No.  '5  rubber  stoppers  on  top  of  which  a  small  piece  of  heavy 
sheet  lead  is  placed,  about  f  in.  thick.  The  level  screws  should 
be  sunk  into  the  lead  about  -fa  in.  deep,  for  the  best  effect1. 

One  source  of  trouble  with  delicate  assay  balances  is  their 
tendency  to  become  magnetized  or  charged  with  static  electricity 
which  will  cause  them  to  act  in  a  very  erratic  manner  during 
weighing.  Balance  beams  constructed  of  material  subject  to 
magnetization  should  be  avoided.  When  a  balance  of  this 
kind  is  in  use  it  may  become  necessary  to  change  its  position  to 
avoid  in  part  the  magnetizing  forces.  For  instance  the  balance 
beam  should  not  be  parallel  to  a  north  and  south  line.  Balances 
constructed  of  non-magnetic  material  may  be  subject  to  similar 

i  D.  M.  Liddell,  Eng.  and  Min.  Jour.,  LXXXIX,  305. 


48 


A    MANUAL    OF   FIRE    ASSAYING 


trouble  due  to  charges  of  static  electricity.  This  is  particularly 
true  in  hot  dry  climates,  and  may  be  accentuated  by  insulating 
the  balance  from  its  surroundings  by  glass  or  rubber  supports. 
When  trouble  of  this  kind  occurs  it  may  be  desirable  to  ground 
the  balance  by  a  copper  wire.1  Balances  which  have  pans  that 
are  blackened  on  one  side  and  are  bright  on  the  other,  seem  to  be 
subject  to  peculiar  disturbance  at  certain  times.  In  this  case 
both  sides  of  the  pan  should  be  blackened. 


FIQ.  40. — PULP  BALANCE. 

It  is  necessary  to  have  an  even  temperature  in  the  balance- 
room  preferably  about  60°  F.  Sunlight  should  be  excluded  if 
possible.  The  balance  must  not  be  exposed  to  a  source  of  heat 
which  will  radiate  unsymmetrically,  otherwise  unequal  expansion 
of  balance-arms  will  cause  incorrect  weights.  In  weighing,  the 
balance-door  should  always  be  closed  to  avoid  the  disturbing 
effect  of  slight  air  currents.  The  true  weight  of  a  mass  can  be 
determined  only  by  correcting  for  the  buoyant  effect  of  air.  The 
error,  however,  is  so  small  that  it  may  ordinarily  be  neglected.2 
Pulp  and  reagent  balances  are  shown  in  Fig.  40.  The  ordinary 
type  of  assay  button  balance  is  illustrated  in  Fig.  41.  Fig.  42 

1  A.  Austin  and  Swift  Hunter,  "Balances,"  M.  and  Sci.  Press,  XCVII,  224. 
2Ostwald,    "Physico-Chemical    Measurements,"    1894,    p.    38.     Ames    and    Bliss,    "A 
Manual  of  Experiments  in  Physics,"  p.  151. 


BALANCES   AND    WEIGHTS 


49 


shows  the  "non-column"  type  of  button  balance.  The  very 
short  column  of  these  balances  by  decreasing  the  length  of  the 
pan  hangers,  tends  to  concentrate  the  movable  mass  near  its 
central  axis  thus  giving  great  stability  of  poise,  while  preserving 
sensitiveness;  the  pointer  extends  upward,  and  the  scale  is  above 
the  beam.  In  some  forms  the  pointer  is  horizontal  and  the  scale 
vertical,  placed  to  the  side  of  the  beam. 


FIG.  41. — ASSAY  BUTTON  BALANCE. 

WEIGHTS. — The  weights  used  in  weighing  beads  are  milligram 
weights,  usually  from  1  mg.  up  to  1000  mg.,  the  units  being  as 
follows:  1,  2,  5,  10,  20,  50,  100,  200,  500  and  1000  mg.  They 
are  best  made  of  platinum,  as  the  material  must  be  not  readily 
corroded,  so  that  the  weight  will  remain  constant.  Riders  are 
used  to  determine  weights  up  to  1  mg.  the  balance-beams  being 
divided  into  100  equal  spaces,  each  space  being  equivalent  to 
0.01  mg.  with  a  1-mg.  rider.  Riders  are  made  of  fine  platinum 
wire,  and  for  assay  balances  usually  come  as  0.5-  and  1-mg.  riders. 
One-milligram  riders  are  commonly  used.  Where  the  balance 
can  readily  be  made  sensitive  to  0.005  mg.,  0.5-mg.  riders  can 
be  used  with  profit;  otherwise  1-mg.  riders  are  preferable,  as 


50 


A    MANUAL    OF   FIRE    ASSAYING 


they  are  not  so  readily  injured  by  handling.  Riders  are  fre- 
quently sold  which  are  not  of  true  weight,  and  it  is  essential  to 
check  them  before  using.  The  same  is  true  of  weights.  It  is 
desirable  for  every  assay  office  to  have  a  set  of  standardized 


FIG.  42. — NON-COLUMN  TYPE  OF  ASSAY   BALANCE.      (Keller.) 


FIG.  43. — PLATINUM  ASSAY  WEIGHTS. 

weights  for  comparison.  These  standardized  weights  can  be 
purchased  from  the  balance  firms;  or  a  set  may  be  corrected  by 
the  Government  Bureau  of  Standards.1 

1  Consult  Circular  No.  3,  U.  S.  Bureau  of  Standards,  Dept.  of  Commerce  and  Labor, 
Washington,  D.  C. 


BALANCES   AND    WEIGHTS 


51 


The  Assay-ton  System. — Gram  and  assay-ton  weights  are 
used  to  weigh  pulp  and  fluxes.  The  assay-ton  system  was  de- 
vised by  Professor  Charles  F.  Chandler,  of  Columbia  University, 
New  York,  and  reconciles  the  difficulties  arising  from  the  fact 
that  all  ores,  etc.,  are  weighed  by  the  avoirdupois  system,  while 
precious  metals  are  weighed  by  the  troy  system.  The  basis  of 


FIG.  44. — ASSAY  TON  WEIGHTS. 


FIG.  45. — GRAM  WEIGHTS. 


the  assay  ton  is  the  number  of  troy  ounces  in  1  ton  (2000  Ib.) 
avoirdupois. 

1  ton  =20001bs.; 

1  Ib.  (avoirdupois)  =7000  troy  grains; 
therefore,  1  ton  =  14,000,000  troy  grains. 

1  oz.  (troy)  =  480  grains; 


therefore,  ~  =  29,  166  oz.  (troy). 


52  A    MANUAL    OF    FIRE    ASSAYING 

Then,  taking  1  mg.  as  the  unit,  1  assay  ton  =  29,166  mg., 
or  29.166  grams,  and  1  mg.  bears  the  same  relation  to  1  assay 
ton  as  1  oz.  troy  bears  to  1  ton  of  2000  Ib.  avoirdupois. 

From  this  it  follows  that  if  1  assay  ton  of  ore  is  taken,  and 
the  silver  and  gold  from  this  is  weighed  in  milligrams,  this  weight 
will  represent  ounces  troy  per  ton  of  ore.  Fig.  43  shows  a  set  of 
platinum  assay  weights;  Figs.  44  and  45  show  a  set  of  assay-ton 
and  gram  weights,  respectively. 


CHAPTER  V 
REDUCTION  AND  OXIDATION  REACTIONS 

REDUCTION.  —  A  reduction  reaction,  as  particularly  denned 
for  assaying,  is  one  in  which  a  metal  is  reduced  from  its  com- 
pounds by  some  reducing  agent.  The  chemical  definition  is  also 
applicable  in  that,  in  assaying,  we  frequently  reduce  a  compound 
from  a  state  of  higher  oxidation  to  a  lower  state  of  oxidation  by 
means  of  a  reducing  agent. 

An  oxidation  reaction  is  "one  in  which  a  metal  or  a  com- 
pound is  changed  to  a  compound  of  a  higher  state  of  oxidation; 
for  example,  Pb  to  PbO,  S  to  SO2,  or  PbO  to  PbO2.  Reduction 
and  oxidation  reactions  frequently  occur  in  assaying,  and  it  is 
essential  that  the  assayer  be  thoroughly  familiar  with  the  theory 
and  facts.  In  speaking  of  reducing  agents  and  reduction  with 
special  reference  to  assaying,  we  have  chiefly  in  mind  such  re- 
agents as  reduce  metallic  lead  from  litharge  in  the  crucible.  The 
chief  of  these  are:  (1)  argol,  (2)  charcoal  or  coke  or  coal  dust,  (3) 
flour  or  sugar.  These  are  added  to  the  charge  in  sufficient  quan- 
tity to  produce  the  proper  size  of  lead  button  in  the  crucible 
assay.  It  often  happens  that  an  ore  will  contain  reducing  agents, 
chiefly  sulphides,  so  that  it  becomes  unnecessary  to  add  an  ex- 
traneous agent.  In  fact,  it  may  contain  an  excess  of  reducing 
agent,  requiring  an  oxidizing  agent  to  destroy  the  excess. 

The  reduction  of  lead  by  argol  is  expressed  by  the  following 
equation: 

10  PbO  +  2KHC4H4O6  =  lOPb  +  5H2O  +  K2O  +  8CO2 
376  2070 

One  gram  of  argol  will  reduce  5.50  grams  of  lead  from  5.93  or 
more  grams  of  PbO.  The  above  formula  for  argol  is  that  of  pure 
bitartrate  of  potassium.  Argol  contains  4as  impurity  a  '  certain 
amount  of  carbonaceous  matter,  so  that  its  reducing  power  will 
be  increased.  It  will  be  found  that  the  actual  reducing  power 
of  1  gram  of  argol  varies  between  7  and  9.5  grams  of  lead,  depend- 
ent on  the  argol  used. 

The  reduction  of  lead  by  charcoal  is  expressed  by  the  following 
reactions: 


12     414 
53 


54  A   MANUAL   OP   FIRE   ASSAYING 

One  gram  of  carbon  will  reduce  34.5  grams  of  Pb.  As  char- 
coal, coal  or  coke  dust  will  contain  more  or  less  inert  ash  which 
has  no  reducing  effect,  the  actual  amount  of  lead  reduced  will  be 
materially  less.  It  will  usually  be  found  to  range  between  20 
and  30  grams  per  gram  of  carbonaceous  reducing  agent  used. 

Flour  will  reduce  from  9  to  12  grams  of  lead  per  gram,  de- 
pending on  the  nature  of  the  flour. 

The  common  sulphides  most  frequently  found  in  ores,  and 
which  give  the  ores  containing  them  reducing  powers,  are  :  Pyrite 
(FeS2),  pyrrhotite  (Fe7S8),  arsenopyrite  (FeAsS),  chalcopyrite 
(CuFeS3),  chalcocite  (Cu2S),  stibnite  (Sb2S3),  galena  (PbS),  and 
sphalerite  (ZnS). 

The  amount  of  lead  reduced  per  gram  of  the  respective  sul- 
phides varies  according  to  the  combination  of  conditions,  which 
will  be  fully  discussed. 

Taking  pyrite  as  an  example,  the  following  equation  expresses 
the  reaction  which  takes  place  when  it  is  fused  writh  soda  and 
litharge: 

(a)  2FeS2  +  15PbO  =  Fe2O3  +  4SO3  +  15Pb 
240  3105 


One  gram  of  pure  pyrite  reduces  12.9  grams  of  lead.     The 
result  can  readily  be  obtained  by  the  following  charge: 
Pyrite  ..........................  .........       3  grams 

Na2CO3  ..................................     10  grams 

PbO  .....................................   100  grams 

The  result  could  not  be  obtained  were  the  pyrite  to  be  fused 
with  litharge  alone,  as  the  presence  of  soda,  a  strongly  alkaline 
base,  induces  the  formation  of  sulphuric  anhydride  (SO3),  which 
combines  with  soda  to  form  sodium  sulphate  (Na2SO4).  This 
sodium  sulphate  will  float  on  top  of  the  slag  and  is  not  decom- 
posed by  the  temperature  usually  attained  in  the  muffle.  It 
separates  out  on  cooling  as  a  fused  white  mass.  Its  melting- 
point  is  885°  C.1  When  the  oxidizing  action  in  the  above  charge 
is  diminished  by  decreasing  the  litharge2  to  below  70  grams, 
the  iron  is  only  partially  oxidized  to  the  ferric  condition  and  the 
two  following  equations  express  the  reactions:3 

1  W.  P.  White.  Am.  Jour.  Set.,  XXVIII,  471. 

2  E.  H.  Miller,  "The  Reduction  of  Lead  from  Litherage,"  in  Trans.  A.  I.  M.  E.,  XXXIV, 
395. 

*  It  must  be  borne  in  mind  that  while  we  speak  of  a  "reducing"  or  an  "oxidizing" 
reaction,  the  reaction  is  really  of  both  natures,  for  while  litharge  is  "reduced,"  the  iron 
pyrite  is  "oxidized." 


REDUCTION    AND    OXIDATION    REACTIONS  55 

FeS2  +  7PbO = FeO  +  2SO3  +  7Pb 
2FeS2  +  15PbO=Fe203  +  4SO3  +  15Pb 

The  first  equation  will  give  12  grams  of  Pb  per  gram  of  pyrite, 
and  the  second  will  give  12.9  grams.  The  accompanying  table 
gives  the  reducing  powers  of  the  various  substances  as  deter- 
mined by  the  litharge-soda  charge  given  for  pyrite. 

TABLE  II.— REDUCING  POWERS  OF  AGENTS 


Name  of  reducing  agent 

Quantity  of  lead  in  grams 
reduced  by  1  gram  of 
reducing  agent 

Argol 

9  61 

Flour                                       .  .                

10.53 

Sugar  

11.78 
26  0 

Sulphur 

18  11     (See  Table  III) 

Pyrite  
Pyrrhotite  
Stibnite            

12.24 

8.71 
7.17 

Chalcocite  

4.38 

Sphalerite 

8  16 

When  no  soda  is  present  to  induce  the  formation  of  alkaline 
sulphates,  the  following  reaction  takes  place,  sulphur  dioxide 
(SO2)  being  formed: 


120  1035 

or  1  gram  of  pyrite  reduces  8.6  grams  of  lead. 

In  the  assay,  as  ordinarily  performed,  the  foregoing  conditions 
are  modified  by  the  presence  of  other  substances,  in  the  main 
by  silica.  Lead  oxide  readily  forms  silicates  with  silica,  and  the 
mono-,  bi-,  and  tri-silicates  are  easily  fusible,  while  those  of  a 
higher  degree  are  fusible  with  difficulty.  When  a  reducing  agent 
(argol,  sulphides,  etc.)  is  fused  with  a  silicate  of  lead,  or  with  a 
charge  containing  litharge  and  silica,  only  a  little  lead  is  reduced 
when  the  silica  is  present  in  amounts  to  form  a  trisilicate  or  above, 
and  only  somewhat  more  when  the  silica  is  present  in  amounts 
to  form  a  mono-  or  bisilicate.  The  reason  for  this  is  that  the 
silicates  of  lead  are  not  reduced  by  sulphides  or  carbonaceous 
reducing  agents  at  temperatures  below  about  1000°  C.1  Above 

1  Consult  MetaUurgie,  IV,  647. 


56  A   MANUAL    OF    FIRE    ASSAYING 

that  temperature  reduction  takes  place  more  readily.  The 
higher  the  silicate  degree  the  more  difficult  is  the  reduction.  If, 
however,  certain  other  bases,  such  as  ferrous  oxide  (FeO),  soda 
(Na2O),  or  lime  (CaO),  are  present  (as  is  the  case  with  most 
ores),  reduction  of  lead  from  the  silicate  occurs,  with  ferrous 
oxide  or  soda,  at  a  comparatively  low  temperature;  but  with 
lime  alone,  only  at  a  high  temperature.  The  following  equation 
expresses  this  condition: 

Pb2SiO4  +  2FeO  +  C  =  Fe2Si04  +  C02  +  2Pb 

No  difficulty  is  encountered  in  reducing  lead  from  the  borates 
of  lead  and  soda,  by  the  ordinary  reducing  agents,  at  1100°  C. 
While  soda  influences  the  amount  of  lead  reduced  from  litharge 
by  the  sulphides  present,  it  has  not  that  influence  on  carbonace- 
ous reducing  agents,  except  in  so  far  as  it  may  reduce  the  acidity 
of  the  charge  and  thus  favor  reduction. 

The  following  charge  gave  results  as  tabulated  below:1 

Reducing  agent 1  gram  Sodium  carbonate ...    10  grams 

Litharge 45  grams  Silica 7  grams 

Pyrite,  in  this  table,  shows  a  reduction  of  9.30  grams  of  lead 
per  gram,  a  figure  to  be  expected  when  its  sulphur  goes  off  partly 
as  SO2  and  partly  as  SO3.  If  the  soda  in  the  preceding  charge 
is  increased,  the  lead  button  will  approach  the  maximum  re- 
ducible by  pyrite. 

TABLE  III.— REDUCING  POWER  OF  AGENTS 


Name  of  reducing  agent 

Quantity  of  lead  reduced 
by  1  gram  of  reducing 
agent 

Argol.  .  . 

9.6 

Flour.. 

10  92 

Sugar.  .. 

11  74 

Charcoal 

26  08 

Pyrite 

9  30 

Sulphur 

18  IP 

NOTE. — Compare  Table  II  with  this. 

1  "The  Reduction  of  Lead  from  Litharge,"  Trans.  A.  I.  M.  E.  XXXIV,  395- 

2  Due  to  the  ready  distillation  of  sulphur,  this  figure  is  difficult  to  obtain;  1  gram  of 
sulphur  will  usually  reduce  6  or  8  grams  of  lead. 


REDUCTION    AND    OXIDATION    REACTIONS  57 

When  carbonaceous  reducing  agents  are  used  to  obtain  the 
required  lead  button,  the  nature  of  the  charge,  as  regards  acidity 
(due  to  SiO2  or  borax),  has  little  influence  on  the  size  of  button, 
provided  sufficient  bases,  outside  of  PbO,  are  present  to  decom- 
pose lead  silicates  formed,  and  the  silicate  degree  does  not  exceed 
a  monosilicate.  The  amount  of  litharge  present  has  some  in- 
fluence. The  quantity  of  carbonaceous  reducing  agent  remaining 
constant,  the  size  of  button  will  increase  somewhat  with  increas- 
ing amounts  of  PbO  in  the  charge.  When  the  reducing  agent  is  a 
sulphide  (often  a  natural  constituent  of  the  ore),  the  acidity  of 
the  charge  influences,  to  a  certain  extent,  the  size  of  button 
obtainable.  It  is,  however,  the  amount  of  alkaline  base  present 
(K2O,Na2O)  that  exerts  the  most  powerful  influence,  its  presence 
inducing  the  formation  of  SO3  and,  consequently,  sulphates,  thus 
reducing  larger  amounts  of  lead  than  when  no  alkaline  bases  are 
present,  the  sulphur  going  off  as  SO2. 

OXIDATION. — Oxidation  of  impurities  in  ores  is  frequently 
necessary  in  order  to  obtain  good  results  in  the  assay.  When 
ores  contain  an  excess  of  sulphides,  arsenides,  etc.  (by  an  excess 
is  meant  a  quantity  above  that  which  will  give  the  required  size 
of  lead  button),  an  oxidizing  agent  is  required  to  oxidize  this 
excess,  enabling  it  to  be  volatilized  or  slagged.  Oxidation  of 
impurities  is  accomplished  in  one  of  two  ways. 

1.  By  the  addition  of  potassium  nitrate  (KNO3)  to  the  charge 
(or  other  oxidizing  agents). 

2.  By  roasting  the  ore,  thus  using  the  oxygen  of  the  air  for 
the  oxidation  of  impurities. 

When  niter  is  added  to  an  assay,  it  reacts  with  the  most 
easily  oxidizable  compound  in  the  charge,  which  is  usually  the 
reducing  agent,  i.e.,  the  sulphide  present.  Extraneous  reducing 
agents,  such  as  argol,  flour,  or  charcoal,  are  present  simulta- 
neously with  niter  only  when  it  is  desired  to  determine  the  oxi- 
dizing power  of  niter  against  these  reagents.  For  the  sake  of 
convenience,  the  oxidizing  power  of  niter  is  expressed  in  terms 
of  lead.  If  finely  divided  lead  is  fused  with  niter,  the  fusion 
reaching  a  temperature  of  1000°  C.  after  one-half  hour,  the  fol- 
lowing reaction  takes  place,  approximately: 

7Pb+6KN03  =  7PbO  +  3K20  +  3N2  +  402; 

or  1  gram  of  niter  oxidizes  2.39  grams  of  lead.  The  actual  num- 
ber of  grams  of  lead  oxidized,  determined  by  a  considerable  num- 
ber of  experiments,  has  been  found  to  be  2.37.  The  analysis  of 


58  A    MANUAL    OF    FIRE   ASSAYING 

the  gas  caught  from  the  fusion  showed  10.75  per  cent,  oxygen,  the 
balance  being  nitrogen.  Oxides  of  nitrogen  were  absent.  This 
indicates  that  when  niter  is  used  in  the  crucible  fusion,  oxygen 
is  evolved  which,  under  certain  conditions,  may  escape  from  the 
charge  without  reaction.  As  already  stated,  the  niter  will  react 
with  the  reducing  agent;  expressing  its  oxidizing  power  in  terms 
of  lead  is  merely  for  convenience.  In  certain  types  of  charges, 
i.e.,  those  containing  litharge,  niter,  and  reducing  agent,  or 
litharge,  soda,  niter,  and  reducing  agent,  practically  theoretical 
results  may  be  obtained;  e.g.,  the  oxidizing  power  of  niter  as 
compared  to  charcoal  is  expressed  by  the  following  equation: 


or  1  gram  of  niter  oxidizes  0.15  gram  of  carbon. 

Taking  the  reducing  power  of  pure  carbon  as  34.5  grams  of 
lead,  the  oxidizing  power  of  niter  against  carbon,  expressed  in 
terms  of  lead,  is  0.15X34.5,  or  5.17  grams.  Ten  fusions  of  a 
charge  composed  of  85  grams  PbO,  1  gram  charcoal,  3  grams 
KNO3,  with  5  grams  PbO  as  a  cover,  gave  very  concordant 
results,  and  showed  the  oxidizing  power  of  niter  to  be  5.10. 
The  reducing  power  of  the  charcoal  was  determined  by  five 
fusions  with  the  same  charge,  omitting  the  KNO^1  These 
results,  of  course,  can  also  be  obtained  by  an  impure  charcoal, 
for,  taking  one  which  has  a  reducing  power  of  26  grams  of  lead 

9A  n 

(this  was  used  in  the  above  fusions),  it  then  contains  —  —  or 

34.5 

0.765  gram  pure  carbon.  If  3  grams  of  niter  have  been  added 
to  the  charge,  the  available  carbon  for  reduction  will  be  0.765  — 
(3X0.15)  or  0.315  gram,  which  will  reduce  34.5X0.315,  or 
10.75,  grams  of  lead.  The  oxidizing  power  of  niter  expressed  in 
lead,  then,  is 

26-10.75 

—  -  -  ,  or  5.12  grams. 
o 

Considering  a  sulphide  and  niter,  and  it  is  in  this  connection 
that  niter  is  almost  invariably  used,  the  following  reaction  takes 
place  in  the  litharge-soda  charge  already  mentioned: 
6KNO3  +  2FeS2-Fe2O3+SO3+3K2SO4+3N2 

SO3  +  Na2CO3=Na2SO4+CO2 

or  1  gram  of  niter  oxidizes  0.39  gram  of  pyrite.  In  the  litharge- 
soda  charge,  1  gram  of  pyrite  reduces  12.22  grams  of  lead;  there- 

>  Thia  finding  confirms  that  of  E.  H.  Miller,  in  Trans.  A.  I.  M.  E.,  XXXIV,  395. 


REDUCTION  AND  OXIDATION  REACTIONS  59 

fore,  1  gram  of  niter  in  this  instance  would  oxidize  12.22x0.39, 
or  4.76,  grams  of  lead.  The  accompanying  table1  shows  actual 
results  obtained  for  the  oxidizing  power  of  niter  against  different 
reducing  agents. 

TABLE  IV.— OXIDIZING  POWER  OF  NITER 


Reducing  agent 


Oxidizing  power  of  niter 
in  terms  of  lead 


Pyrite !  4.73  grams 

Charcoal ]  5.15  grams 

Flour I  5 . 09  grams 

Argol !  4.76  grams 


It  follows,  therefore,  that  the  oxidizing  power  of  niter  varies 
with  the  reducing  agent  used. 

When  the  assay  charge  contains  silica  and  borax  glass,  the 
above  figures  no  longer  hold,  for  in  their  presence  oxygen  is 
evolved  by  the  niter,  which  escapes  from  the  charge,  as  in  the 
case  of  the  oxidation  of  metallic  lead  by  niter.  The  amount  of 
oxygen  lost  (thus  reducing  the  oxidizing  power  of  niter)  is  prob- 
ably a  function  of  the  rate  of  rise  of  temperature,  but  evidence 
also  points  to  the  fact  that  silica  reacts  with  the  niter,  setting 
free  oxygen,  at  a  temperature  very  close  to  that  at  which  niter 
reacts  with  charcoal,  or  at  which  oxygen  will  react  with  carbon. 
Niter  fuses  at  339°  C.,  but  does  not  give  off  oxygen  when  fused 
alone  until  530°  C.  is  reached.  Charcoal  ignites  at  temperatures2 
ranging  from  340°  C.  to  700°  C.,  depending  upon  the  temperature 
at  which  it  was  burnt,  while  silica  begins  to  react  with  niter  at 
very  nearly  450°  C.,  probably  according  to  the  following  reaction: 
2KN03  +  SiO2  =  K,SiO3  +  5O  +  N2 

Thus,  during  the  period  in  which  the  temperature  in  the 
crucible  gradually  rises  to  a  yellow  heat  (that  of  the  muffle), 
oxygen  escapes  during  the  range  from  400°  C.  to  500°  C.,  etc., 
this  last  being  taken  as  an  average  temperature  at  which 
charcoal  will  begin  actively  to  oxidize.3 

llbid. 

2  From  a  number  of  experiments  by  the  author,  willow  charcoal  was  found  to  begin 
reaction  with  niter  at  very  close  to  440°  C. 

*  This  is  offered  tentatively,  as  an  explanation  of  what  occurs. 


60  A    MANUAL    OF    FIRE   ASSAYING 

Niter  will  begin  to  react  with  argol  and  pyrite  at  practically 
its  melting-point. 

The  oxidizing  power  of  niter  against  charcoal  in  charges 
containing  silica  will  frequently  vary  between  3.7  and  4.2  grams 
of  lead,  averaging  about  4  grams.  This  is  1.1  grams  lower  than 
in  the  litharge*-soda  charge.  The  oxidizing  power  of  niter 
against  sulphides  is  but  little  lowered  by  the  presence  of  silica  or 
borax  glass.  When  the  oxidizing  power  of  niter  against  pyrite 
(sulphides)  is  considered,  and  expressed  in  terms  of  lead,  the 
varying  reducing  power  of  suphides  in  different  charges  has  to  be 
taken  into  account.  Taking  as  an  example  a  charge  containing 
considerable  silica,  sf  that  a  large  part  of  the  soda  (alkaline  base) 
is  absorbed  as  a  silicate,  leaving  but  little  to  form  sulphate 
from  the  oxidation  of  the  pyrite,  it  is  found  that  the  reducing 
power  of  pyrite  is  9  grams  of  lead,  as  already  noted.  In  this 
charge,  niter  will  react  with  pyrite  as  follows: 


or  1  gram  of  niter  oxidizes  0.475  gram  pyrite.  The  oxidizing 
power  of  niter  expressed  in  lead  is  then  9  X  0.475,  or  4.275  grams. 
Actually,  it  will  be  very  little  lower  than  this,  as  but  little  oxygen 
escapes  without  action.  The  actual  figure  obtained  by  experi- 
ment is  very  close  to  4.20. 

It  is  evident  from  this  that  the  oxidizing  power  of  niter  varies 
with  the  type  of  charge  used.  It  ranges,  for  pyrite,  from  about 
4  grams  in  acid  charges  to  4.76  in  basic  charges  (containing  no 
silica).  It  varies  still  more  with  other  sulphides.  It  has  been 
the  practice  of  assayers  in  making  the  niter  fusion  to  run  a  pre- 
liminary assay  in  a  comparatively  basic  charge  (approximately 
the  litharge-soda  type),  and  use  the  figure  obtained  for  the  re- 
ducing power  of  the  ore  in  this  charge  in  calculating  the  amount 
of  niter  for  the  final  fusion,  usually  made  quite  acid.  In  this 
way  discordant  results  are  obtained,  for  both  the  reducing  power 
of  the  ore  and  the  oxidizing  power  of  niter  vary  in  the  different 
charges. 

Supposing  that  the  preliminary  assay  showed  the  reducing 
power  of  a  nearly  pure  pyrite  to  be  12  grams  of  lead  per  gram 
of  ore.  Using  a  0.5  assay  ton  in  the  final  fusion,  on  this  basis 
the  amount  of  lead  reduced  would  be  12X15,  or  180  grams. 
Subtracting  the  weight  of  the  lead  button,  20,  from  this  leaves 
the  equivalent  of  160  grams  of  lead  to  be  oxidized.  Taking  it 


REDUCTION   AND    OXIDATION    REACTIONS  61 

as  the  oxidizing  power  of  niter  in  the  final  charge,  40  grams  of 
niter  would  be  added.  But  in  the  final  charge,  owing  to  its 
acidity,  the  reducing  power  of  the  pyrite  is  but  10  grams  of  lead 
per  1  gram  of  ore,  and  the  total  reducing  power  of  0.5  assay  ton 
is  150  grams.  It  therefore  follows  that  the  final  result  will  show 
no  button.  The  oxidizing  power  for  niter  which  should  have  been 
used  is  -^X4,  or  5.3,  and  31  grams  of  niter  added.  This,  then, 
would  give  approximately  the  proper  sized  button.  As  the 
range  of  reducing  power  for  pyrite  is  from  about  9  to  12.2  grams 
of  lead,  according  to  whether  the  charge  is  acid  and  contains 
little  soda,  or  is  of  the  litharge-soda  type,  the  most  satisfactory 
way  to  determine  the  amount  of  niter  to  add^s  to  have  the  nature 
of  the  preliminary  charge  the  same  as  that  of  the  final  charge, 
and  then  use  the  figure  4  to  4.2  as  the  oxidizing  power  of  niter.1 
The  following  charges  are  recommended  to  determine  oxidizing 
and  reducing  powers: 

PRELIMINARY  ASSAY,  No.  1  PRELIMINARY  ASSAY,  No.  2 

5  grams  of  pyritous  ore  5  grams  of  pyritous  ore 

8  grams  of  SiO2  8  grams  of  SiO2 

100  grams  of  PbO  100  grams  of  PbO 

12  grams  of  Na2CO3  12  grams  of  Na2CO3 

Borax  glass  cover  3  grams  of  KNO3 

Borax  glass  cover 

The  difference  in  weight  of  the  lead  buttons  of  preliminary 
assays  Nos.  1  and  2,  divided  by  3,  will  give  the  oxidizing  power 
of  niter  in  the  type  of  charge  used.  The  weight  of  the  button  of 
preliminary  assay  No.  1,  divided  by  5,  gives  the  reducing  power 
of  the  ore. 

PRELIMINARY  ASSAY,  No.  3 

5  grams  of  pyritous  ore  12  grams  of  Na2CO3 

100  grams  of  PbO  Salt  cover 

It  will  be  noted  that  the  reducing  power  of  the  ore  is  greater 
than  that  obtained  in  preliminary  assay  No.  1.  In  order  to 
determine  the  reducing  power  of  argol  and  charcoal,  make  up 
the  following  charges  in  duplicate: 

PRELIMINARY  ASSAY,  No.  4  PRELIMINARY  ASSAY,  No.  5 

5  grams  SiO2  5  grams  SiO2  .  •. 

60  grams  PbO  60  grams  PbO 

10  grams  Na2CO3      *  10  grams  Na2CO3 

2  grams  argol  1  gram   charcoal   or   coke   or   coal 

Borax  glass  cover  dust 

Borax  glass  cover 
1  This  has  reference  to  pure  dry  KNOs. 


62  A    MANUAL    OF   FIRE   ASSAYING 

In  order  .to  determine  the  oxidizing  power  of  niter  as  com- 
pared to  charcoal,  make  up  the  following  charge  in  duplicate: 

PRELIMINARY  ASSAY,  No.  6 

5  grams  SiO2  1  gram  charcoal,  etc. 

60  grams  PbO  3  grams  KNO, 

10  grams  Na2C03  Borax  glass  cover 

Calculate  results  as  directed  for  niter  in  pyritous  ores. 

Certain  basic  ores  will  have  an  appreciable  oxidizing  power, 
so  that  when  the  usual  amount  of  reducing  agent  is  added  to  the 
charge  to  obtain  a  20-gram  lead  button,  it  is  found  that,  due  to 
the  oxidizing  power  of  the  ore,  the  button  is  deficient  in  size. 
The  oxidizing  ingredients  of  an  ore  are  generally  hematite  (Fe2O3); 
magnetite  (Fe3O4),  and  manganese  oxides;  e.g.,  MnO2.  The  reac- 
tion which  takes  place  is  as  follows: 

2Fe2O3  +  C  =  4FeO  +  CO2 

One  gram  of  Fe2O3  requires  0.037  gram  of  carbon  to  reduce 
it  to  FeO. 

In  order  to  determine  the  oxidizing  power  of  an  ore,  make  up 
the  following  charge,  if  the  ore  consists  mostly  of  base.  When 
considerable  silica  is  present  in  the  ore,  decrease  the  silica  in  the 
charge: 

1  assay  ton  of  ore  15       grams  SiO2 

20  grams  Na2CO3  1 . 5  grams  coal 

90  grams  PbO  Borax  glass  cover 


CHAPTER  VI 
THE  CRUCIBLE  ASSAY;  ASSAY  SLAGS 

In  almost  every  instance,  when  a  crucible  assay  is  to  be  made, 
the  ore  and  the  fluxes  added  are  thoroughly  incorporated  by 
mixing,  so  that,  theoretically  at  least,  every  particle  of  the  ore 
is  in  contact  with  a  particle  or  particles  of  fluxes  and  reducing 
agent,  the  most  favorable  condition  to  produce  a  thorough  reac- 
tion among  them.  The  separation  of  the  precious  metals  is 
dependent  upon  their  affinity  for  metallic  lead,  forming  an  alloy 
of  lead,  gold  and  silver,  in  which  lead  greatly  preponderates, 
and  which  readily  settles  by  gravity  from  the  balance  of  the 
ore  and  fluxes  which  have  united  to  form  a  slag.  The  ore  to  be 
assayed  must  in  all  instances  be  in  a  finely  crushed  condition, 
varying  in  American  practice,  from  80-mesh  up  to  200-mesh 
material.  What  takes  place  within  the  crucible  depends  upon 
some  or  all  of  the  following  factors: 

1.  The  fineness  of  crushing.     Are  all  the  particles  of  gold 
and  silver  or  their  alloy  present,  entirely  set  free  from  the  in- 
closing gangue?     In  some  ores  this  takes  place  with  much  coarser 
crushing  than  in  others.     In  other  ores  the  metals  are  so  finely 
disseminated  that  all  are  not  set  free  within  the  limits  of  crushing 
as  carried  out. 

2.  The  mode  of  occurrence  of  the  gold  and  silver.     Is  it  in 
the  free  state,  as  is  most  generally  the  case  with  gold,  or  are  the 
precious  metals  in  the  form  of  a  more  or  less  complex  mineral 
compound  (tellurides,  argentite,  etc.),  which  must  be  decomposed 
before  the  gold  and  silver  will  alloy  with  the  lead? 

3.  The   physical   properties   of   the   slag   produced;   e.g.,    its 
formation  point,  its  fluidity  at  temperatures  somewhat  above  its 
formation  point,  and  its  fluidity  after  superheating. 

4.  The  chemical  nature  of  the  slag,  its  acidity  or  basicity, 
the  nature  of  the  bases  present,  more  particularly  copper,  zinc, 
antimony,  manganese,  iron,  etc. 

If  a  crucible  be  broken  open  and  its  contents  examined  shortly 
after  fusion  has  commenced,  these  will  be  found  to  consist  of  a 
63 


64  A   MANUAL    OF    FIRE    ASSAYING 

heterogeneous  mass  through  which  are  scattered  innumerable 
particles  of  lead,  both  microscopic  and  macroscopic.  The  larger 
particles  have  been  formed  by  the  coalescence  of  the  smaller  par- 
ticles gradually  settling  through  the  charge  toward  the  bottom 
of  the  crucible  to  form  the  final  lead  button  as  the  temperature 
rises  and  the  charge  becomes  more  fluid  and  less  resistant.  It  is 
evident  that  the  completeness  of  the  collection  of  the  precious 
metals  "depends  upon  the  main  factors  already  outlined.  The 
temperature  at  which  carbon  begins  to  react  with  PbO  to  form 
Pb1  is  530  to  555°  C,,  well  below  884°  C.,  the  melting-point  of  PbO. 
The  formation  point  of  a  borate  silicate,  PbO,  Na2O,  4SiO2,  2B2O3 
(Seger  Cone  No.  0.022)  the  constituents  of  which  are  contained 
in  nearly  all  assay  charges,  is  590°  C. 

In  the  fusion  of  a  mixture  containing  silica,  various  bases 
and  borax  glass,  that  silicate-borate  having  .the  lowest  formation 
point  will  form,  and  then  as  the  temperature  rises  absorb  either 
silica  or  base  or  both,  as  these  are  in  excess  of  the  ratio  required 
to  form  the  lowest  formation-point  compound.  If  the  temper- 
ature does  not  rise  high  enough  to  cause  this  absorption,  the 
excess  of  silica  or  base  or  both  will  remain  in  suspension  in  the 
formed  silicate-borate,  practically  in  an  unaltered  condition.  If 
the  formed  silicate,  etc.,  constitutes  the  greater  part  of  the  mass, 
there  will  be  an  imperfect  non-homogeneous  slag;  if  the  excess 
of  silica  or  base  forms  the  greater  part  of  the  material,  there  will 
be  a  slightly  fritted  mass. 

Taking  the  simplest  case,  and  also  the  most  uncommon,  that  of 
an  ore  containing  free  gold  completely  liberated  by  crushing,  the 
particle  of  lead,2  formed  at  a  comparatively  low  temperature,  can 
unite  at  once,  as  soon  as  formed,  with  the  gold  particle  not  in- 
closed in  gangue  and  commence  settling  to  the  bottom  to  form  the 
lead  button.  It  is  evident  that  in  this  instance  the  homogeneous 
fusion  and  chemical  decomposition  of  the  ore  are  immaterial. 
Taking,  however,  the  far  more  common  case,  in  which  the  metals 
are  not  completely  liberated  by  crushing,  it  is  evident  that  the 
particle  of  gold  still  inclosed  within  the  gangue  cannot  be  reached 
by  the  lead  already  reduced,  and  it  becomes  practically  essential 
to  hold  the  lead  in  place  until  the  ore  particle  containing  the  gold 

1  Doeltz  und  Graumann,  Metatturgie,  IV,  420.     According  to  Roscoe  and  Schorlemmer, 
Treatise  on  Chemistry,  II,  865  (1907),  CO  reacts  with  PbO   to   form   Pb,  at  100°  C.     H 
reacts  with  PbO  to  form  Pb  at  310°  C.     Mostowitsch,  Metallurgie,  IV,  648. 

2  There  will  probably  be  many  particles  of  lead  for  each  gold  particle  present,  so  that 
no  gold  will  escape  for  lack  of  lead. 


THE    CRUCIBLE    ASSAY  65 

is  thoroughly  broken  up  chemically  and  liquefied,  so  that  the 
lead  can  absorb  the  gold.  If  the  lead  settles  through  the  charge 
before  this  decomposition  takes  place,  gold  will  remain  in  the 
slag.  The  only  way  to  control  this  condition  is: 

(a)  By  fine  crushing,  liberating  the  metals  as  completely  as 
possible. 

(6)  By  the  choice  of  a  slag  having  the  proper  physical  proper- 
ties, i.e.,  a  low  formation  point  and  a  viscous  nature  near  the 
formation  point. 

(c)  By  a  comparatively  slow  fusion  during  the  early  stages 
of  the  assay,  to  prevent  as  much  as  possible  the  rapid  settling 
away  of  the  lead  particles  through  the  still  existing  interstices  of 
the  charge. 

Where  compounds  of  the  precious  metals  are  in  the  ore, 
such  as  argentite  (AgaS),  tellurides,  calaverite  and  sylvanite, 
(AuAgTe4),  etc.,  these  are  readily  decomposed  by  the  litharge 
as  follows: 

Ag2S  +  2PbO  =  2Pb  Ag  +  S02 

The  tellurides  will  be  especially  considered  in  Chapter  X,  on 
" Special  Methods  of  Assay." 

ASSAY  SLAGS. — An  assay  slag  from  the  crucible  assay  con- 
sists in  most  instances  of  silicates  and  borates  of  metallic  bases. 
While  usually  of  a  homogeneous  nature,  a  slag  is  rarely  a  chem- 
ical compound.  It  is  to  be  considered  in  most  cases  as  a  com- 
plex "solid  solution,"  this  term  as  applied  here  including  both 
the  crystalline  isomorphous  mixtures,  or  "mixed  crystals,"  and 
the  amorphous  glasses.  As  an  example:  Litharge  with  silica 
forms  certain  silicates  which  are  chemical  compounds,  but  which 
have  not  been  definitely  determined,  though  very  likely  Pb2SiO4 
is  one  of  them,  judging  by  cooling  curves  which  have  been 
taken.1  This  silicate  is  capable  of  dissolving  either  PbO  or  SiO2 
and  forming  homogeneous  "solid  solution"  within  certain 
limits,  the  solid  solutions  in  cases  when  the  silica  contents  are 
above  11.94  per  cent.— rcorresponding  to  Pb2Si04 — being  glasses. 

In  a  similar  way  all  the  common  bases,  Na2O,  K20,  FeO, 
CaO,  MgO,  A12O3,  ZnO  and  MnO  form  silicates  which  are  sol- 
uble in  each  other  when  molten,  and  when  frozen  will  form 
either  complex  isomorphous  mixtures  or  amorphous  glassy 
"solid  solutions."  An  assay  slag  is  therefore  usually  a  complex 

1  Wl.  Mostowitsch,  Metallurgie,  IV,  651.     S.  Hilpert,  Metallurgie,  V,  535. 
5 


66 


A    MANUAL    OF    FIRE    ASSAYING 


"solid  solution."  Boric  acid  and  alkaline  borates  act  similarly 
to  silica,  and  if  borax  is  used  in  the  fusion  the  final  slag  will  be 
a  complex  "solid  solution"  of  silicates  and  borates  of  PbO, 
Na2O,  FeO,  CaO,  etc.,  dependent  upon  the  bases  in  the  ore  and 
the  fluxes  used. 

Silicates  are  defined  in  degree  by  the  ratio  of  oxygen  in  the 
base  to  that  in  the  acid.     The  chemical  classification  is  as  follows: 


TABLE  V.— SILICATE  DEGREES 


Name 

Oxygen  Ratio,  Base  to  Acid  ;         Example 

Orthosilicate  
Metasilicate  
Sesquisilicate  
Bisilicate  

1  to  1 
Ito2 
1  to  3 
1  to4 

MgO.FeO.SiO2 
MgO.CaO.2SiO2 
K2O.Al2O3.6SiOa 
Ca0.2SiO2 

The  metallurgical  classification  is  made  on  the  same  basis, 
i.e.,  oxygen  in  the  base  to  that  in  the  acid,  but  is  somewhat 
different.  It  is  the  one  adopted  in  these  notes. 


TABLE  VI.— SILICATE  DEGREES 


Formula,  RO  (base) 

Name 

Formula,  R2O3  (base) 

4RO.  SiO2  
2RO.  SiO2  
4RO.  3SiO2  

Subsilicate 
Monosilicate 
Sesquisilicate 

4R2O3.  3SiO2 
2R2O3.  3Si02 
4R2O3.  9SiO2 

RO.    SiO2 

R  O3     3SiO 

2RO.  3SiO2 

Trisilicate 

2R  O3  9SiO 

Borates  may  be   classified  in  a  somewhat  similar  manner. 

In  general,  it  may  be  stated  that  the  higher  the  silicate  degree, 
the  more  infusible  is  the  mixture,  and  that  a  polybasic  mixture, 
one  of  many  bases,  is  more  easily  fusible  than  one  of  few.  These 
general  statements  are  not  without  exceptions,  for  certain  bi- 
silicates  and  trisilicates  have  a  lower  fusing  point  than  the  corre- 
sponding monosilicate,  etc.  It  also  depends  greatly  upon  the 
base  what  the  fusibility  of  the  silicates  will  be.  PbO,  Na2O,  and 


THE    CRUCIBLE    ASSAY  67 

K20  give  easily  fusible  silicates;  FeO  and  MnO  give  compara- 
tively readily  fusible  silicates;  A12O3,  CaO,  and  MgO  give  diffi- 
cultly fusible  silicates.  When,  however,  silicates  of  all  these  vari- 
ous bases  are  mixed  and  go  into  solution  as  a  homogeneous  mass, 
the  effect  of  this  mixture  on  the  melting-point  of  the  mass  is 
often  to  lower  it.  In  fact,  the  silicate  mixtures  are  to  be  looked 
upon  from  the  same  point  of  view  as  metallic  alloys;  there  may 
be  eutectic  mixtures,  i.e.,  mixtures  of  two  or'more  constituents 
which  have  a  lower  melting-point  than  either  of  the  constituents, 
as  is  illustrated  in  the  accompanying  diagram.1 

'  Rhodonite  Hypersthene 

2  pln.0)  2  (Sl.Oj)  2  (Fe  O)  2  (81  O8j 


1100°C 


1000C 


985°C 


Hypersthene 
80  40  60 80 IQfr 


- 1100°C 


1050°  0 


1000  C 


80  60  40  20  0£ 

Rhodonite 
FIG.  46. — FREEZING-POINT  CURVE;  RHODONITE-HYPERSTHENE. 

The  eutectic  mixture,  or  the  composition  of  lowest  melting- 
point  in  the  series  occurs  at  20  per  cent,  hypersthene  (the 
bisilicate  of  iron)  and  80  per  cent,  rhodonite  (the  bisilicate  of 
manganese).  The  melting-point  of  this  mixture  is  985°  C., 
which  is  considerably  lower  than  that  of  either  constituent 
alone.  In  the  series  CaSiO3— Na2SiO3  a  minimum  occurs  in 
the  freezing-point  curve  at  a  composition  of  80  per  cent.  Na2SiO3 
and  20  per  cent.  CaSiO3,  the  freezing  temperature  being  920°  C. 
while  the  freezing  point  of  Na2Si03  is  about  1010°  C.  and  that 
of  CaSiO3  is  1505°  C.2 

Typical  Assay  Slags. — A  slag  of  low  formation  temperature 
and  considerable  viscosity  at  that  temperature  corresponds  to 
Seger  Cone  No.  0.022 -Na2O.PbO.4SiO2.2B2O3,  590°  C.  This 
may  be  written:  PbO.4SiO2.Na2B4O7. 

1  J.  H.  L.  Vogt,  D  e  Silikatschmdzldsungen,  II,  Christiana. 

2  R.  C.  Wallace,  Zeit.  Anorg.  Chem..  LXIII,  2 


68  A   MANUAL    OF    FIRE    ASSAYING 

By  calculation  from  the  atomic  weights  the  following  charge 
will  yield  this  slag: 

PbO 33.3  grams 

SiO2 36.2  grams 

Na2B4O7 30.4  grams 

The  slag,  corresponding  to  Seger  Cone  0.017  and  melting  at 
740°  C.,  may  be  desirable  for  aluminous  ores: 

(Na2O.PbO.Al203.6SiO2.2B2O3),  which  may  be  written  (Na2B4 
O7.  PbO.  A12O3.  6SiO2). 

The  following  charge  will  yield  this  slag: 

Na2B4O7 22.9  grams        A12O3. 11. 5  grams 

PbO   24.9  grams        Si62 40.7  grams 

TABLE  VII.— ASSAY  SLAGS1 


Formula 

Silicate  degree 

Approximate 
temperature 
(Centigrade) 
at  which  fluid 

Remarks 

1.  2NasO.SiO2  !  Monosilicate.  .  .             1070 

Vitreous,  colorless,  trans 

. 

parent. 

2.  NazO.SiOj  

Bisilicate  

1090 

Stony,    white,    crystal- 

line. 

3.  2PbO.Si02  

Monosilicate.  .  . 

1030 

Vitreous,    light    yellow, 

transparent. 

4.  PbO.Si02  Bisilicate  

1050 

Vitreous,     light    yellow. 

transparent. 

5.  NaaO.FeO.SiO.  Monosilicate.  .  . 

1070 

Very  fluid,  stony  black. 

6.  Na2O.FeO.2SiO  Bisilicate  

1070 

Vitreous,  black. 

7.  PbO.FeO.SiOj  Monosilicate.  .  .             1100 

Resinous,  black. 

8.  Na2O.PbO.SiO2  Monosilicate.  .  .             1020 

Vitreous,  yellow-green. 

9.  Na:O.PbO.2SiO2  Bisilicate  j            1030 

Vitreous,  yellow-green. 

10.  2(PbO.FeO.CaO)3SiO2  Monosilicate.  .  .             1110 

Vitreous,  black. 

11.  Na2O.PbO.FeO.CaO.2SiO-  .  .    Monosilicate.  .  . 

1030 

Vitreous,  black,  con- 

tains sq.  crystals. 

12.  Na2O.PbO.FeO.Ca0.4Si02  .  .  i  Bisilicate  

1100 

Vitreous,  black. 

13.  2(Na2O.PbO.CaO)3Si02.  .  .  .    Monosilicate.  .  . 

1090 

Stony,  light  yellow. 

14.  2(Na20.FeO.CaO)3Si02  Monosilicate.  .  . 

1150 

Viscous,     stony,     gray- 

brown. 

15.  2(Na2O.PbO.FeO)3Si05.  .  .  . 

Monosilicate.  .  . 

1030 

Vitreous,  black. 

A  partial  replacement  of  the  silica  by  borax  glass  in  the 
foregoing  slags  will  appreciably  lower  the  formation  points. 

Bases  such  as  FeO,  CaO,  MgO,  MnO,  BaO,  and  A12O3  are 
present  in  greater  or  lesser  quantity  in  almost  all  ores,  and  SiO, 
is  present  in  practically  every  ore,  so  that  such  slags  as  those 

1  Elmer  E.  West,  Laboratory,  S.  D.  School  of  Mines,  1904 

Stony  slags  indicate  incomplete  solution  of  some  of  the  ingredients. 


THE    CRUCIBLE    ASSAY 


69 


outlined  must  necessarily  be  made.  The  easily  fusible  bases 
PbO  and  Na2O  serve  to  lower  the  formation  point  of  the  slag. 
If  it  is  accepted  that  the  composition  of  the  slag  in  the  assay  is 
practically  the  constant  factor,  it  is  evident  that  when  the  ap- 
proximate composition  of  the  ore  is  known,  we  will  add  either 
basic  or  acid  fluxes,  in  such  proportions  as  to  produce  the  proper 
slag  decided  upon.  The  most  desirable  constitution  for  an  assay 
slag  in  general,  is  that  of  a  monosilicate  or  a  sesquisilicate,  some- 
times, but  more  rarely,  a  bisilicate.  If  the  ore  is  basic  a  bisilicate 
may  be  approached,  if  acid  a  monosilicate,  or  even  a  sub-silicate, 
in  order  to  insure  complete  decomposition  of  the  ore. 
The  accompanying  table  will  simplify  slag  calculations: 

TABLE  VIII.— THE  CALCULATION  OF  SLAGS1 
UNIT  MOLECULAR  BASE  RATIO;  E.G.,  PsO:  NA2O:  FEO,  ETC.  =  !:  1: 1 


One  part 
of  base  by 
weight 

Parts  of  other  bases  necessary 

Parts  of  SiO2 
necessary  for 
monosilicate 

Na20 

PbO 

CaO 

A1203 

FeO 

ZnO 

Na20 

1.000 
0.279 
0.862 
1.108 
0.608 
0.780 
0.763 

3.590 
1.000 
3.095 
3.976 
2.181 
2.801 
2.738 

0.903 
0.252 
0.779 
1.000 
0.549 
0.704 
0.689 

1.646 
0.459 
1.419 
1.823 
1.000 
1.284 
1.255 

1.160 
0.323 
1.000 
1.284 
0.705 
0.905 
0.885 

1.311          0.486 
0.365          0.136 
1.130          0.419 
1.452          0.539 
0.7971         0.886 
1.023          0.379 
1.000          0.371 
| 

PbO 

FeO  
CaO  
A1.0,  
CuO 

ZnO  

One  part  by  weight  of    Na2C 
SiOj  requires  to  form     2.07 
the  monosilicate              parts 

PbO 
7.36 
!     parts 

CaO 
1.86 
parts 

A120 
1.14 
parts 

FeO       ZnO 
2.40      2.70 
parts     parts 

CuO 
2.63 
parts 

When  a  bisilicate  is  to  be  calculated,  the  silica  required  for 
a  monosilicate  is  determined  and  then  multiplied  by  two.  Vice 
versa,  when  the  bases  for  the  monosilicate  have  been  calculated 
and  a  bisilicate  is  to  be  formed,  the  bases  must  be  divided  by 
two.  The  same  reasoning  applies  to  other  silicate  degrees. 

1  Based  on  Balling's  table. 


70  A    MANUAL    OF    FIRE    ASSAYING 

Example  of  the  Calculation  of  an  Assay  Slag. — The  problem  is 
to  calculate  a  charge  to  produce  the  following  monosilicate: 
Na2O.PbO.FeO.CaO.2SiO2.  Taking  as  the  unit  10  grams  of 
Na2O,  it  follows  from  the  preceding  table  that  the  weights  of  the 
substances  required  are: 

Na20 10X1          =10.0    grams 

PbO 10X3.59=35.9    grams 

FeO 10X1.16   =11.6    grams 

CaO 10X0.903=    9.03  grams 

The  silica  required  will  be: 

for  the  Na20 10       XO. 486  =  4. 86  grams 

PbO 35.90X0.136  =  4.86  grams 

FeO 11.60X0.419=4.86  grams 

CaO 9. 03X0. 539=4. 86  grams 


Total 19 . 44  grams  SiOa 

The  silica  may  be  determined  by  calculating  it  for  one  base 
and  multiplying  that  figure  by  the  number  of  oxygen  molecules 
in  the  bases  present,  after  having  reduced  the  slag  formula  to  its 
lowest  possible  terms.  Before  making  up  the  charge,  it  is 
essential  to  remember  that  the  Na20  in  this  instance  is  furnished 
in  the  form  of  NaHCO3,  which  contains  approximately  40  per 
cent,  of  Na2O,  and  that  the  FeO  is  furnished  by  an  iron  ore  of 
the  following  approximate  composition: 

Fe2O3,  80  per  cent.;  SiO2,  17  per  cent. 

The  lime  is  furnished  by  limestone,  CaCO3,  practically  pure. 

It  is  also  necessary  to  provide  a  lead  button;  so  extra  litharge 
must  be  furnished.  To  reduce  the  lead,  coal  dust  is  added. 
Some  of  the  coal  will  be  used  up  to  reduce  the  Fe2O3  to  FeO. 
Hence  the  following  calculations  are  to  be  made:  10  grams  Na2O 

10 
are  required;  therefore  -    X 100  =  25   grams  of   NaHCO3   must 

be  added.     PbO  contains  92  per  cent,  of  Pb;  therefore,  in  order 

orv  v/  -I  f\f\ 

to  obtain  a  20-gram  lead  button, =  22  grams  of  PbO 

must  be  added,  in  addition  to  the  35.9  grams  for  the  silicate — 
a  total  of  57.9  grams  of  PbO.  Eleven  and  six-tenths  grams  of 
FeO  are  required.  Fe2O3  consists  of  90  per  cent,  of  FeO  and  10  per 

cent,  of  O2;  and  as  the  ore  is  80  per  cent,  of  Fe2O3,  — ~ — 


THE    CRUCIBLE    ASSAY  71 

=  16.1  grams  of  ore  will  be  required.     The  limestone  contains  54 
per  cent.  CaO;  therefore,  -:  —  --  -  =16.7  grams  of  limestone 

will  be  required. 

The  coal  in  use  has  a  reducing  power  of  20  grams  of  lead  per 
gram  of  coal. 

The  following  reaction  takes  place  between  carbon  and  the 
Fe203. 

2Fe203  +  C  =  4FeO  +  CO2. 

12 

One  gram   of   Fe2O3   requires  -  =  0.037  gram  of  charcoal. 

320 

20  X  100 
But  as  the  coal  used  is  only  —  ~T~T~  =  58  per  cent,  as  strong  as 


charcoal,  the  following  quantity  will  have  to  be  added  to  the 
16.1  grams  of  Fe203  to  reduce  it: 

0.037X16.1X80 

—  =  0.82  gram  coal. 
0.58 

To  this  must  be  added  1  gram  for  the  reduction  of  the  20-gram 
lead  button,  giving  1.82  grams,  of  coal  to  be  added. 

Since  the  iron  ore  contains  silica,  this  is  to  be  deducted  from 
the  silica  calculated.  The  amount  of  Si02  in  the  ore  is  16.1  X  17 
per  cent.  =2.74  grams. 

The  correct  charge  then  is  : 

25      grams  ......  NaHC03  16.7  grams  .....  limestone 

57.  9  grams  ......  PbO  16.7  grams  .....  silica  (19.44-2.74) 

16.  1  grams  ......  Fe2O3  (iron  ore)         1  .82  grams  .....  coal 

Salt  cover 

Following  is  the  calculation  of  the  same  slag,  but  for  a  quartz 
ore  containing  95  per  cent.  SiO2.  The  formula  for  the  slag  is: 
Na2O.PbO.FeO.Ca0.2SiO2.  Taking  as  the  unit  1  assay  ton  of 
ore,  or,  in  round  numbers,  30  grams,  this  will  contain  28.50  grams 
of  SiO2.  These  28.5  grams  are  to  be  divided  into  4  equal  parts 
to  satisfy  the  4  bases  present.  Therefore,  7.1  grams  of  SiO2 
will  go  to  such  an  amount  of  each  base  as  will  form  a  monosilicate. 

7  .  1  grams  SiO2  require  7  .  1  X  2  .  07  =  14  .  7  grams  Na2O 
7  .  1  grams  SiO2  require  7  .  1  X  7  .  36  =  52  .  25  grams  PbO 
7  .  1  grams  SiO2  require  7.  1X2.  40  =  17.  04  grams  FeO 
7  .  1  grams  SiO2  require  7.1X1.  86  =  13.  20  grams  CaO 


72  A   MANUAL    OF   FIRE   ASSAYING 

The  bicarbonate  of  soda  required  is  —  =  37  grams. 

The  PbO  required  is  52.25  +  22  =  74.25  grams  including  the 
lead  button. 

The  FeC03  (siderite)  required  is  —  —  =  27  grams. 

62 

13.20X100 
The  limestone  required  is =  24.4  grams. 

t)TC 

The  complete  charge  is: 

1  assay  ton  ore  27      grams FeCO3 

37  grams NaHCO3          24.5  grams CaCO3 

'74  grams PbO  1      gram coal 

Salt  cover 

In  one  case  the  ore  is  of  a  basic  nature — hematite  and  lime- 
stone (17  grams  of  each),  and  in  the  other  case  it  is  of  an  acid 
nature — quartz;  yet  the  slag  produced  is  the  same  in  both 
cases.  This  brings  out  the  fact  that  the  slag  is  the  constant  and 
that  fluxes  are  added  of  such  nature  and  in  such  quantity,  deter- 
mined by  the  ore,  as  to  produce  a  slag  of  fairly  constant  composi- 
tion. It  is  to  be  noted  that  the  slag  made  in  the  two  assays 
contains  four  bases,  PbO,  Na2O,  FeO,  CaO,  and  that  these  are 
present  in  unit  molecular  base  ratio.  As  a  matter  of  fact,  the 
assayer  rarely  adds  CaO  or  FeO  as  fluxes,  but  when  these  are 
present  in  the  slag,  they  are  derived  from  the  ore.  The  bases 
added  as  fluxes  are  practically  limited  to  three,  PbO,  Na2O  and, 
at  times,  K2O,  so  that  when  an  ore  consisting  chiefly  of  Si02  is 
to  be  assayed,  the  slag  made  will  approximate  a  monosilicate  and 
borate  of  lead  oxide  and  soda. 

The  table  of  assay  slags  given  mentions  only  those  in  which 
the  bases  are  present  in  the  unit  molecular  ratio.  It  is  evident 
that  where  an  ore  is  considered  in  which  numerous  bases  are 
present,  these  are  not  contained  in  the  unit  molecular  ratio,  so 
that  the  formula  of  the  slag  made  will  rather  have  this  general 
form: 

(xPbO,  yNa20,  zFeO,  tMgO)  vSiO2, 

in  which,  for  a  monosilicate,  considering  the  letters  as  oxygen 
coefficients,  x  +  y  +  z  +  t  =  2v.  In  order  to  get  a  slag  of  low  for- 
mation point,  the  coefficients  of  the  more  infusible  bases,  such 
as  CaO,  MgO,  A12O3,  will  have  to  be  materially  smaller  than  those 
of  the  more  fusible  bases,  PbO,  Na2O,  and  FeO. 


THE    CRUCIBLE    ASSAY  73 

In  assay  practice,  it  is  neither  possible  nor  desirable  to  make 
analyses  of  ore  before  assaying  for  gold  and  silver.  The  assayer, 
however,  is  supposed  to  have  a  good  working  knowledge  of 
lithology  and  mineralogy,  which  will  enable  him  to  form  a  correct 
judgment  of  the  contents  of  his  ore  within  fair  limits.  It  will  be 
comparatively  easy  for  him  to  tell  at  once  whether  he  has  lime- 
stone or  dolomite,  or  an  ore  containing  much  limonite  or  hema- 
tite or  the  iron  sulphides;  or  whether  magnesia,  baryta  or  other 
bases  are  present,  and  in  what  general  proportions.  Following 
are  analyses  of  silicious  and  lead-antimonial  ores: 


TABLE  IX.— SILICIOUS  ORES 


No.  1 

No.  2 

No.  3 

No.  4 

No.  5 

No.  6 

Gold  

0.63  oz. 

0.85oz. 

3.35oz. 

2.00oz. 

0.78oz. 

0.90oz. 

Silver  

2.00oz. 

6.08  oz. 

1.75oz. 

0.62  oz. 

l.OOoz. 

per  cent. 

per  cent. 

per  cent. 

per  cent. 

per  cent. 

per  cent. 

Silica  

65.38 

80.00 

80.90 

84.80 

77.38 

93.72 

Iron  

13.40 

7.50 

9.94 

7.50 

3.54 

2.67 

Sulphur  

11.40 

4.40 

4.53 

0.75 

4.42 

0.69 

Arsenic  

0.90 

2.00 

0.29 

0.00 

0.55 

0.02 

Antimony  

trace 

trace      ' 

trace 

trace 

trace 

0.089 

Tellurium  

0.003 

trace 

0.007 

trace 

Zinc  

Copper  

0.02 

0.004 

0.013 

0.008 

trace 

Manganese  

trace 

0.54 

trace 

0.96 

0.082 

Alumina  

5.43 

1.79 

1.70 

1.02 

2.80 

3.53 

Lime  

2.10 

1.70 

0.50 

0.90 

0.56 

Magnesia  

0.20 

trace 

trace 

TABLE  X.— LEAD-ANTIMONIAL  ORES 


No.  1 

No.  2 

No.  3 

Silica  
Ferrous  oxide  
Alumina 

60.1     per  cent. 
5.2     per  cent. 
9  5     per  cent 

. 
57.65  per  cent. 
0.70  per  cent. 
1  40  per  cent 

59.50  per  cent. 
4.60  per  cent. 
9  00  per  cent 

Magnesia  
Lime  ,  
Lead  
Antimony  
Sulphur  

2.68  per  cent. 
'  trace 
10.6     per  cent. 
4.4     per  cent. 
0  .  5    per  cent. 

2.09  per  cent, 
trace 
16.86  per  cent. 
11.84  per  cent. 

3.00  per  cent, 
trace 
10.1     per  ceat. 
7.55  per  cent. 
0  44  per  cent. 

Water  . 

74  A    MANUAL    OF    FIRE    ASSAYING 

TABLE  XI.— HEMATITE  TABLE  XII.— LIMESTONE 


Analysis  of  a  Hematite 


Analysis  of  a  Limestone 


Silica  
Ferrous  oxide  

14  .  20    per  cen 
73  .  68    per  cen 
5.03    per  cen 

Silica  
Alumina  and  ferric  oxide.  .  .  . 

1.94  per  cent. 
0.68  per  cent. 
0  18  per  cent 

53  61  per  cent 

Manganous  oxide.  .  .  . 

0.19    per  cen 
0.  101  per  cen 

Carbonic  acid  
Water 

43.81  percent. 

These  analyses  are  given  to  show  what  the  chief  base  constit- 
uents may  be,  and  how  ores  will  range  from  acid  types  to  basic 
ones.  Whenever  sulphides  are  present,  it  is  to  be  noted  that  the 
oxidation  of  these  leaves  basic  oxides  to  be  fluxed. 

At  times,  instead  of  silicate  and  borate  slags,  it  is  desirable  to 
make  oxide  slags  in  the  crucible  assay.  This,  of  course,  can  only 
be  done  when  silica  is  absent  from  the  ores,  or  when  a  very  large 
excess  of  litharge  is  used  in  the  fusion.  Litharge,  which  melts 
at  884°  C.,  possesses  the  property  of  dissolving  or  holding  in 
suspension  certain  quantities  of  other  metallic  oxides.  These 
slags  are  discussed  in  the  chapter  "  Assay  of  Impure  Ores. " 

The  charge  for  the  monosilicate  of  lead  and  soda  is  (using  the 
unit  molecular  base  ratio) : 

0 . 5  assay  ton  silica  or  quartz  ore 
39      grams  NaHCO3 
55      grams  Pbo 
Borax  glass  cover 

For  the  bisilicate  it  is : 

0 . 5    assay  ton  silica  or  quartz  ore 
20      grams  NaHCO3 
28      grams  PbO 
Borax  glass  cover 

Allowing  for  a  20-gram  lead  button,  the  charges  are : 


No.  1,  Monosilicate 
ore  (quartz), 
0  .  5  assay  ton 

No.  2,  sesquisilicate  (approxi- 
mate) ore  (quartz), 
0  .  5  assay  ton 

No.  3,  bisilicate 
ore  (quartz), 
0  .  5  assay  ton 

Na2CO3 26  grams    Na2CO3 20  grams      NaCO3 14  grams. 

PbO 77  grams    PbO 60  grams      PbO 50  grams. 

Coal 1  gram  |  Coal 1  gram     i  Coal 1  gram 

Borax  glass  cover        |   Borax  glass  cover  Borax  glass  cover 


THE    CRUCIBLE    ASSAY  75 

All  of  the  above  charges  will  yield  satisfactory  slags  in  an 
ore  assay  if  the  ore  is  of  the  nature  described.  No.  3  is  the 
cheapest  in  point  of  cost;  No.  2  is  the  one  most  frequently  made. 

Color  of  Slags. — Most  slags  from  ore  assays  will  be  from 
light  to  very  dark  green  in  color  or  almost  black,  this  color  being 
due  to  various  proportions  of  ferrous  silicate.  When  iron  is 
absent,  the  color  of  lead  silicates  (yellow)  may  predominate,  or 
white  and  gray  or  colorless  slags,  due  to  silicates  of  CaO,MgO,ZnO, 
etc.,  be  produced.  Copper  produces  red  slags,  due  to  cuprous 
silicate.  Cobalt  gives  blue  slags.  When  much  lime  is  present 
in  an  ore,  this  is  best  calculated  to  a  bisilicate  or  even  higher, 
while  the  other  bases  can  be  calculated  to  the  monosilicate. 


CHAPTER  VII 
CUPELLATION 

Cupellation  has  for  its  object  the  oxidation  of  the  lead  in 
the  gold,  silver,  etc.,  alloy  to  PbO,  which  in  part  (98.5  per  cent.) 
is  absorbed  by  the  cupel,  and  in  part  (1.5  per  cent.)  volatilized. 
The  silver  and  gold  of  the  alloy  are  left  as  a  metallic  bead.  The 
process  is  carried  out  in  cupels.  Cupels  are  shallow  porous  dishes, 
made  generally  of  bone-ash,  or  magnesia,  produced  by  calcining 
magnesite.  Portland  cement  may  be  used  as  a  cupel  material. 

Leached  wood-ashes  (particularly  from  beech-wood)  and  lime 
and  magnesia  have  also  been  used  for  cupels.  A  mixture  of 
bone-ash  and  leached  wood-ashes,  in  the  proportion  of  1  to  2 
and  2  to  1  respectively,  has  been  used,  and  is  said  to  give  a  much 
smaller  absorption  of  the  precious  metals  than  bone-ash  cupels.1 

Bone  Ash  Cupels. — The  bone  which  yields  the  bone-ash  on 
calcining  has  the  following  composition.2 


Sheep  bones 


Cattle  bones 


Ca3(POJ2 !  62.70  per  cent. 

CaCO3 ' 7 . 00  per  cent. 

Mg3(PO4)2 |  1 .59  per  cent. 

CaF2 2 . 17  per  cent. 


58.30  per  cent. 

7.00  per  cent. 

2.09  per  cent. 

1.96  per  cent. 


Organic  matter !     26.54  per  cent.     '     30.58  per  cent 

These  bones  will  produce  bone-ash  of  the  following  composition : 


No.  1  No.  2 


84  .  39  per  cent.  83  .  07  per  cent. 

CaCO,  ........................  ;       9  .42  per  cent.  10  .  00  per  cent. 

CaF2  .........................  ,       4.05  per  cent.  3.88  per  cent. 

|       2  .  15  per  cent.  2  .  98  per  cent. 


1  Kerl,  Probir  Kunst,  1886,  p.  91. 
*  Hemtz,  Erdmari&Jour.  fur  P.  Chem.,  XLVIII,  24. 
76 


CUPELLATION  77 

The  bone-ash  used  for  cupels  must  be  specially  treated  by 
washing  with  an  aqueous  solution  of  ammonium  chloride  (this 
salt  to  the  extent  ol  2  per  cent,  of  the  weight  of  the  bone-ash  to 
be  treated).1  This  reacts  with  CaCO3  and  any  CaO  present,  con- 
verting them  into  CaCl2,  which  is  removed  by  washing  with  water. 
The  presence  of  CaC03  is  very  undesirable  in  bone-ash  for  cupels, 
as  it  begins  to  give  off  CO2  at  800°  C.,  about  the  temperature  of 
the  beginning  of  cupellation,  causing  a  serious  spitting  of  the  lead 
button,  which  entails  a  loss  of  the  precious  metals.  Cupels  should 
not  be  kept  where  the  nitrous  fumes  from  parting  can  be  absorbed 
by  them,  as  these  will  form  Ca(N03)2  with  any  CaO  that  may 
be  present,  which  also  is  decomposed  about  the  temperature  of 
cupellation.  Bone-ash  melts  at  about  1450°  C.  (Hempel). 

The  physical  nature  of  the  cupel,  especially  as  regards  porosity, 
is  very  important.  For  this  reason  there  should  be  a  careful  ad- 
justment  of  the  relative  amounts  of  different  sized  particles 
present.  Practically,  only  the  fraction  of  1  per  cent,  of  the 
bone-ash  should  remain  on  a  30-mesh  screen.  If  there  is  an 
insufficiency  of  fine  particles  in  the  bone-ash,  the  cupel  will  be 
too  porous  and  cause  a  relatively  heavy  absorption  of  gold  and 
silver.  If  the  bone-ash  is  too  fine,  the  cupels  made  from  it  will 
be  too  dense,  prolonging  the  cupellation  and  causing  losses, 
mainly  by  increased  volatilization. 

The  following  is  a  screen  analysis  of  the  bone-ash  commonly 
purchased,  but  which  is  rather  coarse : 

Through  a    20-mesh  screen,  100        per  cent. 

On  a              30-mesh  screen,  2 . 90  per  cent. 

On  a              40-mesh  screen,  6 . 40  per  cent. 

On  a              60-mesh  screen,  10 . 04  per  cent. 

On  a              80-mesh  screen,  2 . 00  per  cent. 

On  a            100-mesh  screen,  11 .20  per  cent. 

Through  a  100-mesh  screen,  68 . 88  per  cent. 

Cupels  should  be  as  uniform  as  possible  as  regards  density, 
and  for  this  reason  are  best  made  by  machine,  in  which  a  constant 
pressure  may  be  obtained,  rather  than  by  hand  molds.  Fig.  47 
shows  a  good  type  of  cupel  machine.  Considerable  pressure  may 
be  used,  and  the  cupels  made  quite  firm.  It  is  not  possible  to 
specify  the  proper  condition  in  definite  terms,  but  a  batch  of 
cupels,  after  being  made  up  and  carefully  dried  for  at  least  three 
weeks  or  a  month,  should  be  tested  by  cupeling  a  weighed  quan- 

1  W.  Bettel.  Proc.  Chem.  and  Met.  Soc.  of  S.  A.,  II,  599. 


78 


A    MANUAL    OF    FIRE   ASSAYING 


tity  (200  mgs.)  of  c.  p.  silver  with  20  grams  of  lead  at  the  proper 
temperature,  850°  C.,  and  the  loss  noted.  It  should  not  exceed 
from  1.5  to  1.8  per  cent. 

The  bone-ash  to  be  made  into  cupels  is  mixed  with  from  8  to 
12  per  cent,  of  water,  in  which  is  dissolved  a  little  K2C03,  or  to 
which  has  been  added  a  little  molasses  or  stale  beer.  After 
making,  the  cupels  should  be  carefully  and  slowly  dried.  If 


Fia.  47. — CUPEL  MACHINE. 


possible,  cupels  should  be  several  months  old  before  using.  In 
the  Royal  British  Mint  no  cupels  less  than  two  years  old  are 
used  for  bullion  assays. 

If  cupels  are  too  rapidly  dried,  or  have  been  made  up  too  wet, 
they  crack  and  check  when  placed  in  the  furnace  and  make  the 
assays  conducted  in  them  unreliable. 

The  importance  of  good  cupels  cannot  be  overestimated. 
Very  frequently,  inaccuracies  in  the  assays  are  due  chiefly  to  the 
cupel.  The  shape  of  the  cupel  has  some  influence  on  the  loss  of 
precious  metals  by  absorption.  If  the  cupel  is  very  flat  and 
shallow,  so  that  the  molten  lead  covers  a  large  area  and  has 
little  depth,  the  time  of  cupellation  is  decreased  as  the  surface 


CUPELLATION  •          79 

exposed  to  oxidation  is  increased,  but  as  the  absorption  of  precious 
metals  is  probably  a  function  of  the  area  exposed,  it  will  be  large 
in  shallow  cupels.1 

Magnesia  Cupels. — Of  recent  years  the  so-called  "patent" 
cupels  have  come  into  wide  use  especially  in  England  and  South 
Africa  and  to  a  lesser  extent  in  the  United  States.  These  cupels 
are  made  almost  invariably  of  a  magnesia  base.  This  magnesia 
is  produced  by  calcining  crude  Austrian,  Californian  or  Turkish 
magnesite,  and  is  used  largely  in  the  steel  industry  for  basic 
refractory  brick. 

The  composition  is  about  90  per  cent  MgO,  and  10  per  cent,  of 
impurities,  chiefly  CaO,  Fe2O3,  A12O3  and  SiO2.  The  cupels  are  in- 
variably very  hard  and  firm,  of  a  brown  color  and  are  formed  under 
high  pressure.  The  exact  composition  of  the  cupels  is  generally 
a  trade  secret.  Magnesia  cupels  cannot  very  readily  be  made  in 
the  laboratory  like  bone-ash  cupels,  and  in  almost  all  instances 
their  cost  is  higher.  A  number  of  brands  are  on  the  market;  as 
the  Morganite  cupel,  madfe  by  the  Morgan  Crucible  Co.,  Battersea 
Works,  London,  those  made  by  Deleuil,  Paris,  and  the  Mabor, 
Scalite,  Velterite,  Star,  etc.,  brands.  Morganite  cupels,  31.5  m.m. 
top  diameter  (about  1.25  in.)  the  common  size,  cost  $3.35  per 
100  in  St.  Xouis. 

The  properties  of  various  types  of  cupels  are  discussed  in  a 
following  section. 

Portland  Cement  Cupels. — Satisfactory  cupels  may  be  made 
of  ordinary  Portland  cement  provided  the  amount  of  mixing 
water  is  carefully  adjusted.2  The  amount  of  water  should  be 
8  per  cent,  of  the  weight  of  the  cement.  If  less  than  5  per  cent, 
water  is  used  the  cupels  are  too  fragile,  if  20  per  cent,  is  used  they 
will  not  readily  pass  the  cupel  machine.  Upon  heating,  cupels 
with  less  than  5  per  cent,  and  with  more  than  15  per  cent,  water 
cracked  about  the  edges.  Cupels  made  of  one-half  cement  and 
one-half  bone-ash  give  good  results. 

Cement  cupels  are  very  cheap  as  compared  to  bone-ash. 
Cement  will  cost  from  35  cents  to  $1.00  per  100  lb.,  while  bone- 
ash  costs  from  $5.00  to  $8.00  per  100  lb.  Cement  cupels  should 
be  thoroughly  dried  before  use,  otherwise  they  will  develop 
cracks  during  heating. 

1  H.  K.  Edmands,  Eng.  and  Min.  Jour.,  LXXX,  245. 

2T.  P.  Holt  and  N.  C.  Christensen,  "Experiments  with  Portland  Cement  Cupels,"  Eng. 
and  Min.  Jour.,  XC,  560.  J.  W.  Merritt,  "Cement  vs.  Bone-Ash  Cupels,"  Min.  and 
Sci.  Press.  C.  649. 


80  A   MANUAL    OP   FIRE    ASSAYING 

CUPELLATION.1 — When  ready  to  cupel  lead  buttons,  the  cupels 
are'  placed,  empty,  in  the  red-hot  muffle  and  allowed  to  remain 
there  for  about  10  minutes  in  order  to  expel  any  moisture,  or 
organic  matter  present  (if  molasses  water  has  been  used  in 
making  them  up).  If  the  buttons  were  placed  into  the  cold 
cupel,  the  lead  would  melt  before  all  the  remaining  moisture  is 
expelled,  which  would  then  pass  up  violently  through  the  molten 
lead,  causing  what  is  termed  "spitting,"  i.e.,  the  projection  of 
small  lead  particles,  carrying  gold  and  silver  from  the  cupel. 
Some  cupels,  made  from  bone-ash  containing  CaC03,  will  com- 
mence to  spit  after  the  cupellation  has  proceeded  for  some  time 
and  the  temperature  has  risen  to  above  800°  C.  This  can  be 
stopped  by  pulling  the  cupel  to  the  cooler  (front)  part  of  the 
muffle,  although  the  cupellation,  after  spitting,  is  to  be  consid- 
ered unreliable.  When  a  piece  of  wood  or  coal  is  placed  in  the 
muffle  to  "open  up"  lead  buttons,  the  cupels  absorb  gases  at 
times,  wjiich  later  on,  when  the  temperature  rises,  are  again  ex- 
pelled, with  a  spitting  of  the  lead. 

When  the  lead  button  is  put  into  the  hot  cupel,  the  lead 
melts  (326°  C.)  and  is  covered  by  a  gray-black  scum.  If  the  lead 
button  is  practically  pure,  as  it  should  be,  this  black  scum  dis- 
appears when  the  lead  reaches  a  temperature  of  850°  C.  This 
is  called  the  "opening  up"  or  "uncovering"  of  the  lead  button. 
The  molten  lead  then  appears  bright,  begins  to  "drive,"  and 
active  and  rapid  oxidation  commences.  Lead  buttons  should 
uncover  as  soon  as  possible  in  the  muffle.  If  other  and  more 
difficultly  fusible  metals,  such  as  Cu,  Fe,  etc.,  are  present,  the 
temperature  of  uncovering  is  higher  and  the  temperature  re- 
quired for  cupellation  is  higher.  These  foreign  metals  should, 
however,  as  a  general  rule,  be  absent. 

Little  flakes  of  PbO  form  on  the  surface  of  the  molten  lead 
and  slide  down  the  convex  surface  of  the  button,  and  are  absorbed 
by  the  porous  mass  of  the  cupel.  The  process  of  cupellation  is 
dependent  upon  the  relation  of  the  surface  of  the  cupel  to  that 
of  the  molten  lead  alloy  and  the  litharge  which  is  formed  by 
oxidation.  There  is  a  great  difference  between  the  surface 
tension  of  molten  lead  and  litharge  and  while  litharge  can  "  wet " 
the  bone  ash  surface  and  hence  be  absorbed,  molten  lead  cannot 
do  so,  or  only  to  a  very  slight  extent  and  hence  is  not  absorbed. 
In  the  same  manner  metallic  silver  and  gold,  left  on  the  cupel  by 

'The  description  which  follows  refers  in  the  main  to  bone-ash  cupels. 


CUPELLATION  81 

the  oxidation  of  the  lead  will  not  be  absorbed  by  the  cupel. 
As  will  be  noted  further  on  there  is  always  a  loss  of  precious 
metal  during  cupellation,  the  greater  part  of  which  is  caused  by 
absorption  by  the  cupel.  Whether  this  absorption  is  due  to 
some  small  part  of  the  lead  alloy  passing  into  the  cupel,  or  to  an 
oxidation  of  some  silver  with  consequent  absorption  has  never 
been  definitely  determined.  It  is  true  that  the  different  cupel 
materials  and  the  physical  condition  of  the  cupel  as  regards 
porosity  influence  absorption,  the  greatest  factor,  however,  is 
temperature  of  cupellation,  a  comparatively  slight  increase  of 
temperature  causing  a  marked  increase  in  absorption.  Whether 
this  increased  absorption  is  due  to  an  increased  oxidation  of 
the  precious  metal,  or  a  decrease  in  the  surface  tension  of  the 
lead  alloy  is  open  to  question.  This  subject  is  again  referred  to 
on  page  133. 

The  temperature  of  cupellation  is  the  most  important  single 
factor  in  the  operation.  Three  distinct  temperatures  must  be 
considered,  (1)  the  temperature  of  the  cupelling  lead;  (2)  the 
temperature  of  the  muffle,  by  which  is  meant  the  temperature 
of  the  interior  of  a  blank  cupel,  directly  adjoining  the  one  con- 
taining the  lead,  and  (3)  the  temperature  of  the  air  in  the  muffle, 
near  the  cupel.  The  vital  temperature  is  that  of  the  cupel- 
ling lead,  but  as  this  is  difficult  to  measure  except  by  special  appa- 
ratus, the  "muffle  temperature,"  which  always  bears  a  distinct 
relation  to  the  temperature  of  the  cupelling  lead  is  used  hereafter 
in  designating  the  "temperature  of  cupellation." 

The  temperature  of  cupellation  for  pure  lead  buttons  should 
be  850°  C.  to  "uncover"  the  button,  this  may  be  lowered  to 
about  770°  C.  during  the  major  part  of  the  cupellation,  but  must 
be  raised  again  to  about  830°  C.  near  the  end  to  finish  the  opera- 
tion. This  applies  to  bone-ash  cupels.  The  temperature  of  the 
lead  itself  during  cupellation  is  higher  than  that  indicated  by 
the  blank  cupel  near  it,  owing  to  the  rapid  oxidation  of  the 
lead.  This  is  shown  by  the  brighter  color  of  the  lead. 

Any  foreign  metals,  as  Cu,  Sb,  Fe,  Zn,  etc.,  which  are  present 
are  oxidized  (some  by  the  PbO  formed),  and  absorbed  by  the 
cupel,  if  not  present  in  too  large  amounts. 

Zn  +  PbO=Pb  +  ZnO. 

Such  elements  as  Sb,  As,  and  Zn,  when  present  in  the  button, 
are  in  part  volatilized  as  oxides,  and  in  part  absorbed.  When 

6 


82  A   MANUAL    OF    FIRE    ASSAYING 

cupellation  for  silver  is  carried  on,  the  temperature  should  not 
be  above  820°  C.,  in  which  case  crystals  of  litharge  (feathers) 
form  on  the  side  of  the  cupel  toward  the  muffle  mouth.  If  the 
temperature  is  too  low  for  the  cupel  to  successfully  absorb 
practically  all  of  the  PbO,  these  feathers  form  low  down  in  the. 
cupel.  When  the  temperature  is  about  right,  they  form  near 
the  upper  rim  of  the  cupel.  It  is,  however,  to  be  noted  that  the 
draft  through  the  muffle  influences  the  formation  of  feather 
litharge;  i.e.,  if  the  draft  is  strong,  feathers  will  form,  although 
the  temperature  is  somewhat  above  820°  C.  During  cupellation, 
the  door  of  the  muffle  should  never  be  left  wide  open,  but  should 
be  set  slightly  ajar,  so  that  the  cold  air  will  not  strike  directly 
upon  the  cupels.  When  silver  and  gold  are  cupelled  for,  owing 
to  the  higher  melting-point  of  the  silver-gold  alloy,  the  finishing 
temperature  will  have  to  be  860°  C.  at  least. 

As  the  cupellation  proceeds,  the  percentage  of  lead  in  the 
alloy  decreases  and  that  of  Ag  and  Au  increases.  The  litharge 
thrown  off  from  the  center  of  the  button  is  in  larger  specks,  and 
brilliant,  and  the  button  assumes  a  more  rounded  form.  When 
this  phenomenon  appears,  the  cupel  should  be  pushed  back  into 
the  hotter  part  of  the  furnace  or  the  temperature  of  the  furnace 
raised  somewhat.  When  the  last  of  the  Pb  goes  off,  large  buttons 
are  covered  with  a  brilliant  film  of  colors  (interference  colors) 
and  the  button  appears  to  revolve  axially.  The  colors  then 
disappear,  the  bead  becomes  dull,  and  then  again  takes  on  a 
silvery  tinge. 

If  now  the  temperature  of  the  muffle  is  below  that  of  the 
melting-point  of  silver  (962°  C.),  or  below  that  of  the  gold-silver 
alloy  constituting  the  bead,  or  if  the  cupel  be  withdrawn  from 
the  furnace,  the  "blick"  or  "brightening"  or  "flash"  of  the 
bead  takes  place;  i.e.,  the  bead  suddenly  becomes  very  bright, 
at  the  moment  of  solidification,  owing  to  the  release  of  the  latent 
heat  of  fusion,  which  raises  the  temperature  of  the  bead  very 
much  for  a  short  time.  The  bead  has  been  in  a  state  of  surfusion, 
i.e.,  in  a  state  of  fusion  below  its  true  freezing-point,  toward 
the  last  of  the  cupelling  operation;  and  if  it  be  lightly  jarred  or  the 
temperature  allowed  to  drop  still  lower  (by  taking  it  out  of  the 
muffle),  it  suddenly  congeals  and  assumes  a  state  normal  (solid) 
to  the  temperature  existing.  The  release  of  the  latent  heat, 
raising  the  temperature  of  the  bead,  causes  the  brightening. 
The  "brightening"  of  very  small  beads  is  rarely  noticeable. 


CUPELLATION  83 

Silver  and  gold  beads  still  containing  small  amounts  of  Pb  or  Cu 
do  not  brighten  so  noticeably.  If  even  minute  quantities  of 
rhodium,  iridium,  ruthenium,  osmium,  or  osmium-iridium,  are 
present,  buttons  will  not  flash.  Platinum  and  palladium  are 
excepted. 

Silver  beads  after  cupellation,  and  at  the  moment  of  solidi- 
fication, also  "sprout."  According  to  Gay-Lussac  molten  silver 
dissolves  22  times  its  volume  of  oxygen,  at  the  freezing-point. 
Later  researches1  prove  this  practically  correct.  At  1020°  C. 
molten  silver  will  hold  19.5  volumes  of  oxygen  (at  760  mm. 
and  O°  C)  and  at  the  melting-point  somewhat  more.  For 
any  given  temperature  the  oxygen  dissolved  is  proportional  to  the 
square  root  of  the  oxygen  pressure.  In  air  at  760  mm.  pressure 
the  oxygen  has  a  partial  pressure  of  150  mm.  and  the  volume 
of  oxygen  dissolved  by  molten  silver  under  assay  conditions  is 
9.65  volumes  at  the  freezing-point  of  silver.  The  oxygen  is 
dissolved  either  as  monatomic  oxygen  or  as  silver  oxide  (Ag2O), 
in  dilute  solution.  It  is  probable  that  this  silver  oxide,  not 
being  soluble  in  solid  silver  is  dissociated  with  explosive  violence, 
with  the  liberation  of  oxygen,  when  the  silver  solidifies. 

This  oxygen,  suddenly  expelled  when  the  bead  solidifies,  causes 
a  cauliflower-like  growth  on  the  bead.  Small  particles  of  silver 
may  even  be  projected  from  it  and  cause  a  serious  loss.  When 
gold  is  present  in  the  silver  bead  to  the  extent  of  33  per  cent,  or 
more,  sprouting  does  not  take  place.  Silver  beads  containing 
small  quantities  of  Pb,  Cu,  Zn,  Bi,  etc.,  will  not  sprout,  so  that 
if  a  button  does  sprout  it  is  a  sign  of  purity. 

Buttons  below  5  mgs.  in  weight  do  not  sprout  readily;  large 
buttons,  however,  do.  Sprouting  can  be  prevented  by  slow 
cooling  in  the  muffle,  or  by  having  ready  a  hot  cupel  which  can 
be  set,  inverted,  over  the  one  holding  the  bead,  and  withdrawing 
both  from  the  muffle,  thus  cooling  the  bead  slowly.  Sprouted 
beads  are  to  be  rejected  as  an  assay. 

When  cupelling  for  silver  alone,  or  for  silver  and  gold,  it  is 
necessary  to  watch  the  end  of  the  cupellation  carefully,  and  to 
promptly  remove  the  cupel  about  30  seconds  to  1  minute  after 
the  bead  has  become  dull.  A  heavy  loss  of  silver  commences  if 
the  silver  buttons  are  kept  beyond  that  time  in  the  furnace.  If 
silver  is  not  to  be  determined,  but  gold  only,  the  buttons  may 

1  Donnan  and  Shaw,  Jour.  Soc.  Chem.  Ind.,  XXIX,  987.  Sieverts  und  Hagenacker, 
Zeit  Phys.  Chem.,  LXVIII,  115. 


84  A   MANUAL    OF    FIRE    ASSAYING 

be  left  in  for  5  to  10  minutes  without  loss  of  gold.  Gold  beads 
will  retain  minute  amounts  of  lead  which  cannot  be  removed  by 
permitting  the  bead  to  stay  in  the  muffle. 

It  is  to  be  noted,  however,  that  silver  lead  alloys  containing 
between  80  and  90  per  cent,  of  silver  also  show  the  phenomenon  of 
sprouting  or  developing  a  cauliflower-like  growth  on  solidification. * 

The  bead,  when  cold,  is  taken  from  the  cupel  with  a  pair  of 
pliers,  and  cleaned  of  bone-ash  by  flattening  somewhat  with  a 
hammer.  It  should  be  examined  with  a  glass  to  make  sure  that 
no  bone-ash  adheres  to  it. 

The  bead  should  be  either  white  or  yellow,  depending  on  the 
amount  of  gold  present,  round  and  not  flat  (the  latter  indicating 
the  presence  of  foreign  metals),  and  should  possess  a  crystalline 
surface  where  it  adhered  to  the  bone-ash.  It  should  be  firmly 
attached  to  the  bone-ash  of  the  cupel.  If  it  is  not,  this  fact 
indicates  that  lead  is  still  present.  It  should  also  have  no  rootlets 
extending  into  the  cupel.  The  cupel,  after  cupellation,  should 
be  smooth  and  firm,  not  fissured  and  cracked,  and  of  a  light 
yellow  color  when  cold.  Other  colors  indicate  the  presence  of 
foreign  metals. 

The  freezing-point  curve  of  lead-silver  (Fig.  48)  will  give 
some  idea  of  the  proper  temperature  of  cupellation.  A  lead 
button  is  to  be  considered  as  an  alloy  of  lead  and  silver  (or  gold) 
which  in  the  process  of  cupellation  undergoes  the  change  from 
practically  pure  lead  to  that  of  pure  silver  (or  gold). 

A  20-gram  button  containing  200  mgs.  of  silver  contains 
1  per  cent,  of  Ag.  An  alloy  of  lead  and  silver  containing  4  per 
cent,  of  Ag  is  of  "eutectic  composition"  and  melts  at  303°  C., 
the  melting-point  of  pure  lead  being  327°  C.  Most  assay  buttons 
will  contain  very  much  less  than  1  per  cent,  of  silver  and  will 
melt  practically  at  the  melting-point  of  lead.  Leaving  out  of 
consideration  for  the  moment  that  lead  "uncovers"  at  850°  C. 
in  an  oxidizing  atmosphere,  and  the  proper  temperature  required 
to  cause  a  ready  absorption  of  PbO  by  the  cupel,  it  is  evident 
that  for  a  lead  button  weighing  20  grams  and  containing  20  mgs. 
of  silver  (0.1  per  cent.),  the  temperature  required  to  keep  the 
button  molten  ranges  from  327°  C.  to  303°  C.,  until  the  button 
has  decreased  |f  in  weight  by  the  loss  of  Pb,  practically  the 
entire  time  of  cupellation. 

When  the  button  has  reached  ¥V  of  its  original  weight,  the 

»K.  Friedrich,  Metallurgie,  III,  398. 


CUPELLATION 


85 


temperature  required  to  keep  it  molten  will  rapidly  increase, 
according  to  the  curve,  as  more  lead  is  oxidized,  until,  in  order 
to  prevent  freezing  and  get  pure  silver,  a  temperature  of  910°  C. 


1000 


AgO  10  20  30          40  50  60  70          80 

PblOO         9080  70005040  3030 

Fro.  48. — FREEZING-POINT  CURVE,  LEAD-SILVER 


100 
0' 


1000 
900 


700 


1062 


CuO 
PblOO 


30    40     50    60     70    80 

70    60    50     40     30    30 

49. — FREEZING-POINT  CURVE,  LEAD-COPPER 


100 
0 


and  slightly  above  must  finally  be  reached.1  In  order,  however, 
to  cause  a  rapid  formation  of  PbO  and  its  ready  absorption  by 
the  cupel,  and  not  have  heavy  losses  of  Au  and  Ag,  it  is  found 

1  While  the  melting  point  of  silver  is  962°  C.,  this  temperature  is  not  necessary  as  surfusion 
takes  place. 


86  A   MANUAL    OP    FIRE   ASSAYING 

that  a  temperature  of  about  850°  C.  is  best  for  the  main  part  of 
the  cupellation.  It  is  evident,  however,  that  in  order  to  finish 
the  cupellation,  the  heat  must  be  raised  toward  the  end,  other- 
wise the  alloy  of  lead  and  silver,  as  it  increases  in  silver  percent- 
age, will  tend  to  freeze,  i.e.,  to  solidify.  It  is  also  to  be  noted, 
however,  that  this  tendency,  with  most  lead  buttons  of  ordinary 
silver  contents,  is  not  reached  until  very  near  the  end  of  the 
cupellation.  It  is  an  old  saying  amongst  assayers  that  "a  cool 
drive  and  a  hot  blick"  are  essential  to  a  good  cupellation.  In  the 
cupellation  for  silver  it  would  seem  at  first  sight  that  a  final 
temperature  of  962°  C.  is  necessary  in  order  to  prevent  freezing 
and  to  obtain  a  silver  bead  free  from  lead.  However,  the  phe- 
nomenon of  the  "surfusion"  of  the  silver,  i.e.,  silver  in  a  molten 
state  below  its  true  melting-point,  due  probably  to  its  formation 
from  its  lead  alloy  by  the  oxidation  of  the  lead,  appears  to  indicate 
that  this  temperature  is  not  necessary.  It  is  true,  nevertheless, 
that  the  finishing  temperature,  depending  somewhat  upon  the 
amount  of  silver  present,  may  not  fall  much  below  910°  C. 

It  is  plain  that  buttons  may  be  cupelled  at  temperatures  much 
above  those  stated,  but  the  loss  of  silver  and  gold,  both  by  ab- 
sorption and  volatilization,  is  very  much  increased  with  the 
higher  temperatures. 

The  reasoning  outlined  for  silver  applies  also  to  gold,  except 
that,  owing  to  the  somewhat  higher  melting-point  of  gold 
(1063°  C.),  the  finishing  temperature  should  be  a  little  higher. 

It  is  of  interest  at  this  point  to  more  fully  discuss  the  question 
of  temperature  of  cupellation.  This  term  has  been  used  in  a 
vague  manner  by  writers  on  the  subject  and  has  been  used  to 
signify  generally  the  temperature  of  the  air  of  the  muffle,  either 
at  the  side  or  just  above  the  cupel,  or  that  of  the  interior  of  the 
cupel.  Due  to  the  heat  of  combustion  of  the  lead  neither  of 
these  temperatures  is  the  true  temperature  of  cupellation.  The 
actual  temperature  of  cupellation  has  only  recently  been  deter- 
mined,1 due  probably  to  the  fact  that  this  determination 
involves  experimental  difficulties,  since  the  protective  tube  of 
the  thermo  couple  in  almost  any  form  is  rapidly  destroyed  by 
the  corrosive  action  of  the  litharge.  As  already  stated  three 
temperatures  may  be  considered  during  cupellation.  (1) 
The  temperature  of  the  cupelling  lead;  (2)  the  temperature  of 

1  C.  H.  Fulton  and  O.  A  Anderson  and  I.  E.  Goodner,  West.  Chem.  and  Met.,  IV,  31. 
which  consult  for  methods  of  temperature  determinations  of  cupellation. 


CUPELLATION 


87 


the  muffle  as  determined  by  that  of  a  blank  cupel,  adjoining  the 
one  containing  the  lead;  and  (3)  the  temperature  of  the  air 
immediately  surrounding  the  cupel.  This  is  invariably  lower 
than  the  first  two  temperatures,  which  accounts  for  the  low 


FIG.  50. — TEMPERATURE-CURVE  SHOWING  DIFFERENCE  BETWEEN  AIR  IN  MUFFLE 
AND  CUPELLING  LEAD. 

temperature  figures  that  have  been  assigned  to  the  cupellation 
process.  It  is  to  be  noted  that  there  is  an  air  draft  through  the 
muffle  during  cupellation,  cold  air  constantly  entering  at  the 
mouth  of  the  muffle,  so  that  the  air  in  the  muffle  does  not  attain 
the  temperature  of  the  muffle  walls.  The  actual  air  temperature 


88  A  MANUAL  OF  FIRE  ASSAYING 

is  also  probably  somewhat  lower  than  the  thermo  couple  junction 
shows,  since  this  absorbs  heat*  radiated  from  the  muffle  walls 
more  rapidly  than  the  air.  Fig.  50  shows  the  two  temperature 
curves,  one,  the  actual  temperature  of  the  cupelling  lead  and  the 
other  that  of  the  air  in  the  muffle,  close  by  and  at  a  level  with 
the  top  of  the  cupel.  The  general  form  of  the  curve  is  due  to 
fluctuations  of  temperature  in  the  muffle,  caused  by  firing  and 
attempts  to  regulate  the  temperature,  by  draft  and  otherwise. 
It  will  be  noted  that  the  temperature  of  the  cupel  rises  rapidly 
after  oxidation  has  commenced,  attaining  a  maximum  of  940°  C. 
and  then  falling  as  the  muffle  cooled.  The  interesting  data  is  the 
difference  between  the  temperatures  of  the  air  in  the  muffle  and 
the  cupel,  which  is  greatest  during  the  period  of  active  oxidation. 
The  maximum  difference  is  145°  C.  The  lead  "froze"  or  was 
covered  over  with  a  coating  of  PbO,  preventing  further  cupella- 
tion  at  802°  C,  the  air  in  the  muffle  being  then  at  675°  C.  The 
actual  minimum  temperature  of  cupellation  in  this  case  was 
therefore  802°  C.,  127°  higher  than  the  air  temperature.1 

Experiment. — To  determine  the  temperature  of  the  "opening 
or  uncovering"  of  the  button;  i.e.,  the  beginning  of  cupellation, 
and  the  "freezing"  of  the  button;  i.e.,  where  cupellation  is  stopped 
by  the  formation  of  PbO  which  is  not  absorbed. 

In  this  experiment,  134  grams  of  lead  were  used.  The  pres- 
ence of  gold  or  silver  has  no  influence  on  these  critical  tempera- 
tures, as  the  melting-point  of  the  alloys  is  usually  far  below  the 
"uncovering"  temperatures  and  the  precious  metals  form  no 
oxides  which  would  complicate  matters.  The  influence  of  such 
metals  as  copper  will  be  referred  to  further  on.  The  set  was  run 
with  a  blank,  at  the  same  temperature  as  the  cupel  before  the 
lead  was  added.  Fig.  51  gives  the  curves  plotted  as  before.  The 
results  show  that  the  button  begins  to  uncover  at  800°  C.  and 
804°  C.  and  begins  to  "freeze"  at  804°  and  788°  C.  These  are 
the  actual  cupel  temperatures.  A  repetition  of  the  experiment 
in  the  same  cupel  shows  "uncovering"  at  832°  C.,  829°  C.,  and 
834°  C.  and  a  freezing  at  850°  C.  Other  results  show  the  begin- 
ning of  "uncovering"  at  797°  C.  and  completely  open  at  805°  C. 
Another  shows  an  opening  to  occur  at  811°  C.  Another  shows 

1  In  order  to  definitely  prove  the  difference  in  temperature  to  be  due  to  the  oxidation 
of  the  lead,  a  set  was  run  in  which  the  lead  in  the  cupel  was  covered  by  a  clay  dish  luted  on, 
practically  preventing  oxidation.  In  this  instance  the  muffle  and  cupel  were  at  nearly 
the  same  temperature  for  the  space  of  an  hour,  first  one  being  a  little  higher  and  then  the 
other 


CUPELLATION 


89 


S 


S        H 

• 

5        H 

a    § 

8       =0 

3     2 


si  S 


a    § 

!l 


3      £ 


g     SSSSSSSSS8 


90  A    MANUAL    OF   FIRE    ASSAYING 

an  "uncovering"  at  826°  C.;  another  experiment,  at  809°  C. 
In  general,  the  opening  temperatures  and  freezing  temperatures 
are  near  each  other,  as  is  to  be  expected  in  so  far  as  the  two  are, 
in  the  absence  of  any  silica,  practically  the  result  of  the  same 
process.  The  freezing  temperature,  however,  may  be  somewhat 
higher  or  lower  for  a  number  of  reasons  developed  below.  These 
critical  temperatures  are  of  importance  in  so  far  as  they  mark  the 
minimum  possible  temperature  of  the  beginning  of  cupellation. 
From  the  curves  it  will  be  noted  that  as  soon  as  the  button 
uncovers,  there  is  a  sharp  rise  in  the  temperature  of  the  lead 
whether  the  muffle  temperature  rises  or  not,  due  to  the  oxidation 
of  the  lead.  In  the  curves  where  the  muffle  blank  and  the  cupel 
were  at  the  same  temperature  before  the  dropping  in  of  the  lead, 
the  oxidation  raises  the  cupel  temperature  from  20°  to  150°  C. 
above  that  of  the  muffle  dependent  upon  the  rate  of  oxidation, 
i.e.,  the  air  supply.  Since  it  has  been  well  established  that  the 
chief  cause  determining  the  loss  of  precious  metal  by  absorption 
and  volatilization  is  the  temperature,  it  is  at  once  apparent  that 
for  careful  work  the  air  supply  of  the  muffle  is  just  as  important 
as  a  regulation  of  the  temperature  of  the  muffle  itself.  In  Fig. 
51  the  cupel  temperature  does  not  rise  greatly  above  the  muffle 
temperature.  This  is  due  to  the  fact  that,  just  as  soon  as  the 
lead  had  opened,  the  furnace  was  again  cooled  in  order  to  get  a 
determination  of  the  temperature  of  the  "freezing,"  thus  pre- 
venting the  attainment  of  maximum  oxidation. 

What  determines  the  "uncovering"  and  "freezing"  of  the 
buttons?  It  would  appear  at  first  sight  that  the  critical  tem- 
perature of  "uncovering"  and  "freezing"  is  the  melting-point  of 
litharge,  in  so  far  as  the  melting  of  the  cover  of  oxide  and  its 
absorption  by  the  cupel  would  naturally  mark  the  "opening." 
Recent  and  accurate  determinations  of  the  melting-point  of  pure 
litharge,  give  906°  C.1  and  884°  C.,2  with  the  latter  probably  the 
figure  to  be  preferred.  In  these  researches  it  is  noted  that  be- 
fore the  melting-point  is  reached  there  is  a  decided  soft  and 
pasty  stage,  which  is  ascribed  to  the  marked  volatilization  of  PbO 
from  the  solid  state.  This  volatilization  begins  just  below 
800°  C.3  and  is  a  function  of  the  area  exposed  and  the  tempera- 
ture. As  in  the  case  under  consideration,  the  film  of  PbO  on 

1  O.  Doeltz  and  Mostowitsch,  M etallurgie,  IV,  290. 

2  Mostowitech,  M  etallurgie,  IV,  468. 

8  O.  Doeltz  and  C.  A.  Graumann,  Metallurgie,  III,  408, 


'CUPELLATION  91 

the  button  gives  probably  the  greatest  area  in  relation  to  volume 
possible,  this  volatilization  is  an  important  factor  in  the  uncover- 
ing of  the  button,  in  the  case  of  the  absence  of  silica  or  borax, 
and  the  practical  ceasing  of  this  volatilization  marks  the  "freez- 
ing" of  the  button.  It  will  be  noted  that  all  the  temperature 
determinations  of  the  "opening"  and  "freezing"  are  well  be- 
low the  melting-point  of  litharge.  It  is  here  self-evident  that 
when  a  button  is  put  into  a  cupel  whose  temperature  is  900°  C. 
and  above,  that  the  temperature  at  which  "opening"  is  observed 
has  no  special  significance. 

Lead  buttons  from  crucible  assays  practically  always  have 
adhering  to  them  small  amounts  of  silicious  slag,  and  the  bone- 
ash  at  times  contains  minute  quantities  of  SiO2.  When  the  lead 
button  melts  in  the  cupel,  this  slag  and  fine,  loose  bone-ash  go  to 
the  surface  into  the  litharge  film.  According  to  recent  research 
on  the  lead-silicates,1  the  silicates— 6PbO.Si02,  5PbO.SiO2  and 
4PbO.SiO2  are  thinly  fluid  at  794°  C.,  796°  C.  and  726°  C.  respec- 
tively. The  percentage  composition  of  these  silicates  is  as 
follows: 

Silicate  Litharge  Silica 

6  PbO.SiO2  95.68%  4.32% 

5  PbO.Si02  94.86%  5.14% 

4  PbO.SiO2  93.66%  6.34% 

It  is  evident  that  when  the  very  small  amount  of  litharge 
which  forms  the  film  is  considered,  that  minute  quantities  of  sil- 
ica only  are  necessary  to  materially  lower  the  "opening"  tem- 
perature of  the  button.  From  these  facts  it  follows  that  the 
"opening"  or  "uncovering"  temperature  is  not  a  fixed  temper- 
ature, but  will  depend  upon  the  following  factors: 

1.  The   presence   of  silica  in   a   condition  to   combine  with 
lead  (very  probably  also   of  borax).     Where  this  silica  comes 
from  has  already  been  mentioned. 

2.  The  vaporization  of  solid  litharge.     As  the  rate  of  vapor- 
ization  depends   upon   the   temperature,    and   the   relation   of 
area  exposed  to  volume  present,  a  button  with  a  thick  covering 
will  not  open  at  as  low  a  temperature  as  one  with  a  thin  cover- 
ing.    This  can  be  demonstrated  by  placing  a  button  in  the  cupel 
at  a  temperature  below  700°  C.  and  permitting  it  to  form  a  heavy 
film  of  PbO,  then  raising  the  temperature  to  the  usual  "uncov- 

1  Wl.  Mostowitsch,  Melallurgie,  IV,  64.7. 


92  A   MANUAL   OF   FIRE   ASSAYING 

ering"  point,  and  placing  another  button  into  a  second  heated 
cupel.  The  last  button  will  uncover  first,  as  its  thinner  cover  of 
litharge  will  vaporize  in  less  time. 

3.  The  presence  of  foreign  metals  in  the  lead,  such  as  cop- 
per, iron,  etc.,  will  raise  the  "uncovering"  temperature.  This 
is  a  frequently  observed  fact,  and  the  reasons  for  it  are  practically 
obvious.  If  the  temperature  of  the  cupel  at  the  moment  of  un- 
covering could  remain  fixed,  the  increase  in  the  oxidation  of 
the  lead  would  very  soon  balance  the  vaporization  at  that  tem- 
perature and  the  button  would  again  freeze,  but  it  has  already 
been  noted  that  a  very  sharp  rise  in  temperature  at  once  occurs 
automatically;  i.e.,  independent  of  the  muffle,  due  to  the  rapid 
oxidation  of  the  lead ;  this  effects  a  marked  increase  in  vaporiza- 
tion, keeping  the  button  open  and  soon  in  most  instances  the 
temperature  of  the  button  itself  passes  to  and  beyond  the  melt- 
ing point  of  litharge  (884°)  and  cupellation  proceeds  rapidly. 
Cupellation,  however,  can  seemingly  be  carried  on  below  the 
melting-point  of  litharge,  as  Figs.  51  and  52  will  show.  The 
particles  of  litharge  formed  on  the  surface  of  the  button,  though 
solid,  are  pasty  and  capable  of  being  asborbed  by  the  cupel, 
or  the  surface  of  the  cupelling  lead  being  the  area  of  the  most 
active  oxidation  is  at  or  above  the  temperature  of  melting 
litharge,  which  the  thermo-j unction  at  the  bottom  of  the  lead 
does  not  indicate.1 

In  one  experiment,  containing  considerable  silver,  it  was 
noted  that  very  near  the  end  of  the  cupellation  when  the  amount 
of  silver  was  large  and  that  of  lead  small,  the  button  was  cupel- 
ling at  an  indicated  cupel  temperature  of  750°  C.,  the  button  then 
solidified  and  proved  to  be  a  lead-silver  alloy.  The  temperature 
of  750°  evidently  did  not  represent  the  surface  temperature  of 
the  button  as  was  indicated  by  the  brightness  of  the  PbO  specks 
formed;  i.e.,  the  amount  of  heat  liberated  by  the  small  amount 
of  lead  oxidized  was  insufficient  to  make  any  material  impression 
on  the  thermo-j  unction. 

The  "Freezing"  of  the  Button. — When  the  temperature  of 
the  muffle  falls  so  that  the  heat  of  oxidation  of  lead  is  no  longer 

1  In  an  experiment  to  shed  light  on  this  point,  70  grams  of  pure  PbO  were  placed  in  a 
cupel  and  heated  to  815°  C.  for  the  time  of  20  minutes.  The  litharge  showed  vaporization , 
but  none  was  absorbed  by  the  cupel.  In  a  duplicate  experiment  the  temperature  was 
raised  to  883°  C.  just  below  the  melting-point,  and  while  the  litharge  did  not  melt,  all  of  it 
was  rapidly  absorbed  by  the  cupel.  In  the  first  case  the  mass  of  litharge  was  sintered. 
Absorption  thus  probably  occurs  in  the  "pasty"  stage  mentioned. 


CUPELLATION  93 

enough  to  keep  its  temperature  such  that  the  rate  of  vaporization 
is  in  excess  of  the  rate  of  oxidation,  the  molten  button  will  be- 
come covered  by  a  film  of  litharge  and  cupellation  ceases. 

"Feathers"  are  crystals  of  solid  litharge  sublimed  from  the 
vapor  and  deposited  on  the  cupel  walls.  That  the  cupel  walls 
are  invariably  cooler  than  the  cupelling  lead  is  self-evident. 
Feathers  will  therefore  form  when  the  temperature  of  the  cupel 
wall  is  below  or  near  that  of  "  uncovering. "  They  will  not  form 
above  about  820°  C.  The  cooler  wall  and  that  on  which  feathers 
most  usually  form  is  that  toward  the  muffle  mouth,  due  to  the 
direct  impingement  of  cooler  air  currents.  These  feathers  form 
the  best  guide  to  the  temperature  of  cupellation  ordinarily 
available.  During  their  formation  the  actual  temperature  of  the 
cupelling  lead  is  usually  from  840°  to  900°  C.,  although  it  may  be 
appreciably  higher  if  the  oxidation  be  rapid.  The  rapidity  of 
the  oxidation  depends  largely  on  the  air  supply,  and  this  heavy 
air  current  striking  the  cupel  may  cool  the  walls  sufficiently  to 
cause  a  heavy  sublimation  of  feathers,  although  the  true  temper- 
ature of  cupellation,  i.e.,  that  of  the  button  may  be  unduly  high. 

Resume. — From  the  foregoing,  it  appears  that  the  "uncov- 
ering" of  the  button  occurs  at  from  800°  to  840°  C.  dependent  on 
several  factors,  and  that  the  actual  minimum  temperature  of 
cupellation,  may  be  placed  at  about  850°  C.,  but  usually  rises 
above  this,  i.e.,  independent  of  the  muffle,  frequently  to  930° 
and  940°,  unless  the  muffle  temperature  is  lowered  after  uncover- 
ing. The  necessary  finishing  temperature  is,  however,  higher 
than  850°  C. 

Experiment. — To  determine  the  phenomena  incident  to  the 
"finishing"  of  a  cupellation  containing  silver,  i.e.,  that  of 
"surfusion,"  "sprouting,"  freedom  of  the  silver  bead  from  lead, 
temperature  necessary  to  finish,  etc. 

It  has  frequently  been  noted  that  a  cupellation  containing 
silver  and  gold,  or  both,  could  seemingly  be  "finished,"  i.e., 
all  the  lead  eliminated  therefrom  when  the  temperature  of  the 
muffle  was  well  below  that  of  the  melting-point  of  silver;  i.e., 
962°  C.,  or  that  of  the  gold-silver  alloy.  From  the  foregoing,  it 
is  evident  that  the  temperature  of  the  muffle  is  not  by  any  means 
the  same  as  that  of  the  cupellation.  Roberts-Austen1  quotes 
Dr.  Van  Riemsdijk,  stating  that  "he  observed  that  a  globule  of 

1  An  Introduction  to  the  Study  of  Metallurgy,  5th  Ed.,  p.  50,  citing  Ann.  de  Chim. 
et  Phys.  t.  XX  (1880>,  66. 


94  A    MANUAL    OF    FIRE    ASSAYING 

gold  or  silver  in  a  fused  state  will  pass  below  its  solidifying 
point  without  actually  solidifying,  but  the  slightest  touch  with  a 
metallic  point  will  cause  the  metal  to  solidify  and  the  consequent 
release  of  its  latent  heat  of  fusion  is  sufficient  to  raise  the  globule 
to  the  melting-point  again,  as  is  indicated  by  the  brilliant  glow 
which  the  button  emits."  Rose1  also  quotes  the  same  author, 
and  it  is  evident  that  the  gold  and  silver  globules  mentioned  are 
derived  from  cupellation. 

Six  sets  of  experiments  were  carried  on  in  this  connection, 
some  of  which  are  plotted  in  Figs.  52,  53,  and  54.  It  is  evident 
from  these  Figs,  that  surfusion  unquestionably  occurs,  and  in  a 
most  marked  manner,  the  greatest  degree  of  surfusion  noted 
being  77°  C.  All  of  the  buttons  "  sprouted, "  i.e.,  showed  cauli- 
flower-like growths  of  silver  on  final  solidification.  This  sprout- 
ing has  always  been  considered  a  sign  of  purity  of  the  silver,2 
particularly  pointing  to  the  absence  of  lead. 

In  order  to  test  this  point,  some  of  the  silver  buttons  from  the 
experiments  were  very  carefully  examined  for  lead  in  quantities  of 
a  gram,  and  showed  but  traces  of  it,  quantities  not  determinable. 
Some  showed  minute  quantities  of  copper.  In  effect  they  were  all 
"fine  silver."  The  surfusion  is  therefore  very  real.  In  the 
authorities  cited  on  surfusion,  the  statement  is  made  that  on 
solidification  from  surfusion,  the  "flash"  of  the  button  occurs, 
showing  the  raising  of  the  temperature  to  the  melting-point  of 
the  silver.  H.  M.  Howe3  states:  "Once  freezing  sets  in  (in  the 
surfused  metal  or  alloy)  the  heat  which  it  evolves  raises  the 
temperature  toward,  and  more  often  quite  to,  the  true  freezing- 
point,  where  it  remains  during  the  remainder  of  the  freezing." 

In  experiments  carried  on  with  the  following  quantities  of  silver, 
10,  14,  18,  30.4  and  30  grams,  the  "flash"  was  not  observable, 
neither  by  the  eye  nor  by  any  actual  rising  deflection  of  the 
galvanometer  pointer,  although  a  repeated  and  careful  search 
was  made  for  this.  In  order  to  determine  whether  the  size  of  the 
button  had  any  influence  on  the  "flashing,"  various  amounts  of 
silver,  beginning  with  350  mgs.  and  varying  by  50  mgs.  up  to  850 
mgs.,  were  cupelled  so  as  to  finish  with  surfusion.  It  was  found 
that  the  beads  up  to  and  including  650  mgs.  flashed  markedly, 
that  of  700  mgs.  faintly  only,  and  those  above  showed  no  "  flash. " 

1  Metallurgy  of  Gold,  4th  Ed.,  p.  598. 

2  Rose,  Metallurgy  of  Gold,  p.  477.     Collins,  Metallurgy  of  Silver,  1900,  p.  2.     Schnabel, 
Metall-Huettenkunde,  1901,  p.  605,  2nd  Ed. 

3  Iron,  Steel  and  Other  Alloys,  1903,  p.  20. 


CUPELLATION 


95 


If  the  differences  in  temperature  between  the  cupelling  alloy 
and  the  muffle  blank  at  any  time  interval  be  plotted  as  ordinates 
from  a  basal  line,  it  is  readily  shown  by  the  different  curves,  that 
the  greatest  difference  occurs  at  the  close  of  the  cupellation;  in 
some  instances,  just  as  the  last  of  the  lead  oxidizes  (play  of 
colors).  The  differences  noted  show  the  marked  evolution  of 
heat  at  the  "finishing"  of  the  cupellation,  and  are  due  to  the 
release  of  the  latent  heat  of  fusion.  In  the  case  of  the  large 


Difference  Carve 


FIG.  52. — CURVE  SHOWING  TEMPERATURE  DURING  CUPELLATION  OF  Pb  =  Ag. 

buttons,  however,  this  does  not  seem  to  be  sufficient  to  cause  an 
actual  rise  of  temperature  in  the  cupel,  when  the  muffle  tempera- 
ture is  actively  sinking,  as  was  the  case  in  experiments  shown 
by  Figs.  52,  53  and  54.  As  already  stated,  however,  no  "flash" 
was  observable  to  the  eye  in  the  larger  silver  buttons,  nor  did  the 
galvanometer  indicate  it,  as  surely  might  be  expected.  The 
"lag"  or  time  interval  between  the  occurrence  of  a  temperature 
and  its  recording  by  the  galvanometer,  is  not  great  when  an 
iron  protective  tube  is  employed.  This  is  shown  very  plainly  by 


96 


A    MANUAL    OF    FIRE   ASSAYING 


Note:- 

Constant  difference  betwi 
Cupel  and  Muffle  Blan!s=2000 

<  18  Grams,  Ag. 

1  85      -         Pb. 


od      eJ 

8    1 
I    3 


Difference  Curve 
Basal  Line 


FIQ.  53. — CunvE  SHOWING  TEMPERATURE  DURING  CUPELLATION  OF  Pb  =  Ag. 


Note:- 

Shortly  before  addition  of  Alloy, 

Temperatures  of  Cupel  and  Muffle 

uk  were  ideuttc 
10  Grams,  Ag. 
Pb. 


0248   8101214101820222426283332313838404244484850525458580062646868707274767880 
Miuute* 

Fia.  54. — CURVE  SHOWING  TEMPERATURE  DURING  CUPELLATION  OF  Pb  =  Ag. 


CUPELLATION  97 

the  marked  "jogs"  in  the  cupel  curves  when  the  lead  alloy  is 
added  to  the  cupels  (Figs.  53  and  54).  The  lowest  "finishing" 
temperature  found  showed  surfusion  extending  to  about  885°  C. 
or  77°  C.  below  the  melting-point  of  silver.  The  temperature 
indicated  by  the  "muffle  blank,"  which  at  the  start  was  10° 
be.low  the  cupel,  was  845°  C.  The  last  of  the  lead  went  off  from 
the  cupel  at  910°.  This  represents  about  the  minimum  "finish- 
ing temperature,"  judging  by  the  general  appearance  of  the 
cupellation.  It  is  to  be  noted,  however,  that  this  "finishing 
temperature "  is  reached  automatically  in  the  cases  where  the 
muffle  temperature  is  such  as  to  afford  "uncovering"  of  the  but- 
ton and  prevention  of  freezing;  i.e.,  approximately  830°  to  840°  C. 
on  the  average,  in  the  case  .of  pure  lead  buttons.  One  experi- 
ment was  carried  out  in  which  the  cupelling  lead  alloy  showed  a 
temperature  of  75001  not  far  from  the  end  of  cupellation,  but  at 
this  temperature  the  button  solidified  into  a  lead-silver  alloy. 
The  approximate  composition  of  the  silver-lead  alloy  freezing 
at  this  temperature  is  70  per  cent.  Ag,  30  per  cent  Pb. 
Resume. — It  appears  from  the  foregoing  that: 

1.  In  the  case  of   the  lead  buttons  not   containing  any  ap- 
preciable amount  of  copper  or  iron,  etc.,  a  muffle  temperature  of 
at  least  800°  C.  and,  better,  one  of  850°  is  required  to  "uncover" 
or  start  cupellation. 

2.  That  this  temperature  may  be  lowered  to  about  770°  C. 
during  the  oxidation  of  the  greater  part  of  the  lead. 

3.  That  toward  the  end  of  the  cupellation  or  the  "finishing," 
in  case  of  silver,  it  must  again  be  raised  to  about  830°  C.  in 
order  to  get  a  pure  silver  button. 

4.  That  the  actual  temperature  of  the  cupelling  lead  is  always 
appreciably  higher  than  the  muffle  temperature. 

5.  That  the  actual  finishing  temperature  of  the  cupellation 
cannot  safely  be  carried  below  about  910°  C. 

6.  That  the  greatest  observed  surfusion  of  silver  was  77°  C.  and 
that  this  is  probably  very  near  the  maximum. 

7.  That  silver  beads  finishing  with  surfusion  are  free  from  lead. 

8.  That  "feathers"  or  crystals  of  sublimed  litharge  on  the 
cupel  are  an  indication  of  the  proper  cupellation  temperature, 
provided  the  air  draft  is  not  excessive. 

9.  That  it  is  just  as  essential  to  regulate  the  air  draft  of  the 
muffle  as  its  temperature. 

»For  an  explanation  of  this  seemingly  low  cupellation  temperature  see  p.  92. 
7 


98  A   MANUAL    OF    FIRE   ASSAYING 

AVhere  very  accurate  cupellation  work  is  required,  such  as  in 
bullion  assaying  and  where  the  amount  of  work  justifies  it,  a 
furnace  designed  for  close  temperature  and  air  control  is  practically 
essential.  In  view  of  the  recent  improvement  in  electrically 
heated  furnaces,  in  which  temperatures  can  be  rapidly  and  ac- 
curately controlled,  and  the  muffle  heated  uniformly,  practically 
eliminating  the  thermal  gradient,  a  furnace  of  this  type  would 
seem  best  adapted  for  the  work. 

INFLUENCE  OF  BASE  METAL  IMPURITIES.— When  the  lead 
buttons  are  contaminated  with  base  metals,  such  as  copper, 
the  temperature  of  cupellation  must  be  higher  in  order  to  pre- 
vent freezing.  The  reason  for  this  is  readily  apparent  when 
the  freezing-point  curve  (see  Fig.  49)  of  the  lead-copper  series 
of  alloys  is  inspected.  The  freezing-point  of  an  alloy  contain- 
ing 10  per  cent.  Cu  and  90  per  cent.  Pb  is  900°  C. 

While  the  original  copper  percentage  in  the  lead  button  may 
be  quite  small,  the  copper  does  not  oxidize  as  readily  as  the  lead, 
and  tends  to  concentrate  in  the  button,  rapidly  raising  the 
melting-point  of  the  alloy. 

For  the  removal  of  copper  in  cupellation  the  ratio  of  Pb  to 
Cu  should  be  at  least  200  to  1  or  more.  Even  then  Cu  will  be  re- 
tained by  the  silver  and  gold  in  small  amounts.  If  it  is  less  than 
this  considerable  copper  is  very  apt  to  be  retained  with  the 
silver  and  gold.  In  order  to  cupel  at  all,  the  ratio  of  Pb  to  Cu 
must  be  at  least  20  to  1.  In  general,  buttons  to  be  cupelled 
should  be  free  from  base  metal  impurities.  If  they  are  unavoid- 
ably present  in  the  button  from  the  crucible  assay,  the  base  metals 
should  be  removed  by  scorification  before  cupellation. 

Impurities  in  lead  buttons  are  detected  by  the  behavior  of 
the  button.  Zn,  As,  Sb,  and  S  tend  to  make  the  button  brittle 
when  hammered;  iron  and  copper,  etc.,  tend  to  make  it  hard. 
PbO  in  the  lead  button  makes  it  brittle.  PbO  is  often  found  in 
lead  buttons  that  have  been  produced  at  too  low  a  temperature. 
Where  the  gold  and  silver  contents  of  the  lead  button  approach 
30  per  cent,  of  its  weight,  it  is  brittle. 

However,  impurities  in  the  lead  button  will  not  always  be 
indicated  by  brittleness  or  hardness;  without  these  characteristics, 
impurities  may  still  be  present  in  sufficient  amount  to  cause  loss. 
All  impurities  do  not  cause  like  amounts  of  loss  in  cupellation. 
The  loss  due  to  the  presence  of  impurities  is  chiefly  in  absorption 
by  the  cupel,  and  comparatively  small  by  volatilization. 


CUPELLATION 


The  accompanying  table1  shows  the  influence  of  impurities. 
Twenty-five-gram  lead  buttons  were  cupelled,  containing  1  gram 
of  the  impurity  specified,  4  mgs.  of  Ag,  and  1  mg.  of  Au.  The 
temperature  of  cupellation  was  1000°  C.,  in  order  to  prevent 
freezing  as  a  result  of  impurity. 

The  high  losses  are  due  in  part  to  the  high  temperature  em- 
ployed. The  table  really  gives  the  relative  influence  of  the  im- 
purities. Bismuth  has  been  used  in  place  of  lead  for  cupellation. 
While  in  the  table  bismuth  is  stated  to  be  the  cause  of  a  very 
heavy  absorption,  this  is  not  substantiated  by  other  researches.2 
When  it  is  present  in  the  lead  button  it  tends  to  concentrate  dur- 
ing the  cupellation,  and  is  removed  by  oxidation  toward  the  last 
of  the  operation.  Some  of  it  is  very  apt  to  be  retained  by  the  pre- 
cious metal  bead.  Cupellation  may  be  carried  on  with  bismuth, 
but  the  absorption  is  much  higher.3  The  presence  of  Bi  in  the 
cold  cupel  may  be  recognized  by  the  fact  that  the  place  which 
the  silver  button  occupies  is  brown  and  surrounded  by  concentric 
rings  of  a  yellow  and  blackish-green  color.  Copper  colors  the 
cupel  from  a  dirty  green  to  a  black,  dependent  on  the  amount 
of  copper. 

TABLE  XIII .--INFLUENCE  OF  IMPURITIES 


Impurity 

Loss  of  Gold 

Loss  of  Silver 

Remarks 

None  
Tin  

1  .  2  per  cent. 
2  .  0  per  cent. 

11  .8  per  cent. 
13  .  9  per  cent. 

Arsenic  
Antimony.  
Zinc  
Cadmium  
Iron  
Manganese  
Molybdenum  
Vanadium  
Copper 

3  .  9  per  cent. 
5.3  per  cent. 
9  .  3  per  cent. 
3  .  5  per  cent. 
4.0  per  cent. 
13.6  per  cent. 
11.0  per  cent. 
7  .  7  per  cent. 
10  0  per  cent 

16.3  per  cent. 
13.3  per  cent. 
17.6  per  cent. 
13.1  per  cent. 
16.6  per  cent. 
24  .  3  per  cent. 
26.2  per  cent. 
21  .  7  per  cent. 
32  6  per  cent. 

Most  of  this 
loss,  even  with 
Te  and  Se,  is 
cupel  absorp- 
tion. 

Bismuth4 

21  8  per  cent. 

27  9  per  cent. 

Thallium  
Tellurium  .  .  .  .'  
Selenium  

23  .  1  per  cent. 
55.8  per  cent. 
54  .  1  per  cent. 

34  .  4  per  cent. 
67  .  9  per  cent. 
64  .  5  per  cent. 

• 

1  T.  K.  Rose,  in  Jour.  Chem.  Met.  and  Min.  Soc.  of  S.  A.,  Jan  ,  1905. 
2K    Sander,  Berg-  und  Huettenmaennische  Zeitung,  1903,  p.  81.     See  also  Min.  Ind., 
XII,  244. 

3  Smith,  in  Jour.  Chem.  Soc.,  1894,  863. 

4  Doubtful. 


100 


A    MANUAL    OF   FIRE    ASSAYING 


Tin,  arsenic,  zinc,  cadmium,  iron,  and  manganese  cause  scoria 
to  form  on  the  cupel,  due  to  the  formation  of  oxides  which  are 
not  readily  absorbed.  Iron  causes  a  dark  coloration  of  the  cupel. 
Antimony  in  considerable  quantity  causes  the  cupel  to  check 
and  crack.  The  same  may  be  said  of  copper. 

Copper. — This  metal  is  oxidized  with  more  difficulty  than  lead, 
the  Cu2O  forming  by  aid  of  the  action  of  PbO;  however,  Cu2O, 
again  coming  into  contact  with  metallic  lead,  is  reduced  to  Cu, 
and  in  this  way  is  persistent  towrard  the  end  of  the  cupellation, 
although  a  large  excess  of  Pb  over  Cu  is  present,  and  finally  some 
remains  with  the  Au  and  Ag.  The  loss  of  silver  during  the  cupel- 
lation is  due  mainly  to  absorption,  in  large  part  as  oxide.  This 
oxidation  of  the  silver  in  the  presence  of  much  lead  is  not  to  be 
ascribed  to  the  action  of  atmospheric  oxygen,  but  rather  to 
"oxygen  carriers,"  such  as  PbO,  Cu2O,  etc.  It  is  very  probable 
that  Cu2O  acts  peculiarly  in  this  manner,  and  the  high  absorption 
noticed  when  Cu  is  present  is  due  to  this  fact.  It  is  to  be  noted 
that  losses  in  silver  occur  toward  the  end  of  the  cupellation,  and 
occur  in  great  part  just  before  finishing;  the  small  black-green 
rings,  surrounding  the  place  where  the  silver  bead  rests,  locates 
most  of  the  silver.  It  is  the  concentration  of  the  copper,  silver, 
and  gold  that  causes  the  high  absorption.  Lodge1  shows  the 
influence  of  small  amounts  of  copper  on  the  cupellation  of 
silver  and  gold. 

TABLE  XIV.— COPPER  IN  CUPELLATION  OF  SILVER  AND  GOLD 


Silver 
milli- 
grams 


Lead  ;  Copper 
grams     grams 


Percentage  Temperature   Percent- 

of  copper     degrees  cen-     age  of 

in  lead     \      tigrade2  loss 


Ratio  Pb 
toCu 


202 

10 

'0.0101 

0.1 

775 

1.05 

1000  to  1 

203 

10 

0.0202 

0.2 

775 

1.08 

500  to  1 

202 

IP 

0.0303 

0.3 

775 

1.29 

333  to  1 

202 

10 

0.0404 

0.4 

775 

1.45 

250  to  1 

204 

10 

0.0500 

0.5 

775 

Cu  re- 

200 to  1 

tained 

1  "Notes  on  Assaying,"  p.  143  et  seg. 
3  Temperature  of  air  in  muffle. 


CUPELLATIOX 


101 


Gold 

milli- 
grams 

T      ,     Percentage  Temperature 
of  copper     degrees  cen- 
in  Pb            tigrade1 

Percentage  of  loss 

Ratio  Pb 
toCu 

202 

10 

no. 

775 

0.155 

202 

10 

0.1 

775 

0.  192  All  contained  '   1000  to  1 

201 

10 

0.2 

775 

0.20  copper  on             500  to  1 

200 

10 

0.3 

775 

0.13  finishing                333  to  1 

201 

10 

0.4 

775 

0.165 

250  to  1 

202 

10 

0.5 

775 

0.250                              200  to  1 

Gold  is  more  retentive  of  copper  than  silver.  It  is  to  be  noted 
that  even  with  a  ratio  of  200  Pb  to  1  Cu,  it  is  not  possible  to 
remove  all  copper,  and  beads  obtained  from  mattes  and  heavy 
copper  ores  should  be  examined  for  copper;  otherwise  silver 
results  may  be  high.  Retained  copper  in  these  silver  beads  will 
compensate  for  loss  of  silver,  but  the  amount  retained  is  so  vari- 
able that  this  error  cannot  be  considered  to  compensate  the  loss. 

Tellurium. — Tellurium  has  a  great  affinity  for  gold  and  silver, 
and  if  present  in  an  ore  in  any  appreciable  amount,  some  of  it 
will  go  into  the  lead  button  with  the  gold  and  silver,  and  thus 
have  its  influence  on  the  cupellation.  It  tends  to  concentrate 
during  the  cupellation  and  is  with  difficulty  removed  by  oxidation. 
When  there  is  present  in  the  lead  button  more  than  15  per  cent, 
of  the  gold  and  silver  weight  in  tellurium,  the  beads  resulting 
from  cupellation  have  a  dull  and  frosted  appearance.  Larger 
amounts  than  this  cause  the  beads  to  divide  and  split  up  in  the 
cupel.  F.  C.  Smith3  shows  the  influence  of  tellurium  on  the 
cupellation  as  follows,  these  results  being  confirmed  by  J.  C. 
Bailar4  and  others. 

1  Temperate  of  air  in  muffle. 

2  Actual  losses;  copper  retained,  0.16  per  cent.     Gold  about  the  same  weight  as  before 
cupellation. 

3  "The  Occurrence  and  Behavior  of  Tellurium  in  Gold  Ores,"  etc.,  in  Trans.  A.  I.  M.  E, 
XXVI,  495. 

4  "West.  Chem.  and  Met,"  I,  119. 


102 


A    MANUAL    OF    FIRE    ASSAYING 


TABLE  XV.— TELLURIUM  IN  CUPELLATION  OF  GOLD  AND 
SILVER 


Mgs.  of 
bullion 

Containing 

Mgs.Te 
added 

Loss  by  absorption 

Loss  by  volatili- 
zation 

Au 

Ag 

Au 

Ag 

Au 

Ag 

29.8 

24.76 

5.04 

5.0 

per  cent. 
13.44 

per  cent. 
27.08 

per  cent. 
5.65 

per  cent. 
0.69 

28.45 

23.64 

4.81 

15.0 

34.22 

35.78 

5.28 

1.75 

22.17 

18.42 

3.75 

15.  01 

29.85 

32.01 

11.92 

17.95 

CUPELLED  WITH  12  GRAMS  OP  LEAD 

Note  the  similar  effect  of  selenium. 

Antimony. — The  presence  of  antimony  causes  increased  losses 
by  absorption,  although  its  effect  is  not  as  pronounced  as  that 
of  copper  or  tellurium.  During  the  cupellation  litharge  and 
antimony  combine  to  form  antimoniate  of  lead,  which,  if  present 
in  considerable  amount,  may  cause  the  formation  of  scoria  on 
the  cupel.  Small  amounts  of  antimony  tend  to  remain  with  the 
gold  and  silver,  as  with  copper  and  tellurium. 

As  a  guide  in  cupellation,  the  following  scale  of  color  temper- 
atures is  given.2 

Degrees  Centigrade 

Lowest  red  visible  in  the  dark 470 

Dark  blood-red  or  black-red 532 

Dark  red,  blood-red,  low  red 566 

Dark  cherry-red 635 

Cherry-red,  full  red 746 

Light  cherry,  light  red 843 

Orange 900 

Light  orange 941 

Yellow 1000 

Light  yellow 1080 

White 1205 

CUPELLATION  IN  CUPELS  OF  DIFFERENT  MATERIAL.— The 

cupel  material  has  a  decided  influence  on  the  progress  of  a  cupella- 
tion.    What  has  preceded  refers  more  particularly  to  bone-ash 

1  Selenium  instead  of  tellurium. 

2  White    and   Taylor,    in    Trans.    Am.    Soc.  Mch.  Eng.,  XXI,  628.     H.  M.  Howe,  in 
Eng.  and  Min.  Jour.,  LXIX,  75. 


CTJPELLATION  103 

cupels.  In  cupels  with  a  magnesia  base  the  process  as  regards  tem- 
perature differs  somewhat,  due  to  the  different  thermal  properties 
of  the  two  types  of  material.  The  following  difference  in  thermal 
properties  may  be  noted.1  Bone-ash  cupel,  mean  specific  heat 
between  15°  and  100°  C.  is  0.185.  Magnesia  cupel,  mean  specific 
heat  for  same  temperatures  is  0.215.  A  bone-ash  and  magnesia 
cupel  of  identical  volumes  weigh  respectively  22  and  29  grams. 
The  heat  conductivity  of  magnesia  cupels  is  very  much  greater 
than  that  of  bone-ash  cupels.  When  the  two  types  of  cupels  are 
heated  to  90°  C.  in  a  steam  bath,  at  the  end  of  14  minutes  the 
magnesia  cupels  are  at  90°  C.  and  the  bone-ash  cupels  at  only 
60°  C.  During  cupellation  of  lead  at  the  end  of  6  minutes  from 
the  addition  of  the  button  the  magnesia  cupel  showed  practically 
the  same  temperature  in  the  cupelling  lead  as  in  the  bottom  of  the 
cupel,  viz.  920°  C.,  while  the  bone-ash  cupel  in  the  same  muffle 
showed  a  temperature  of  990°  C.  for  the  cupelling  lead,  and  only, 
932°  C.  in  the  bottom.  The  total  heat  capacity  of  a  magnesia 
cupel  is  more  than  50  per  cent,  greater  than  that  of  a  bone-ash 
cupel  of  the  same  volume,  so  that  on  cooling  the  two  types  of 
cupel  the  magnesia  cupel  retains  a  higher  temperature  somewhat 
longer  than  the  bone-ash  cupel  in  spite  of  its  greater  diffusivity 
of  heat.  From  this  data  the  reason  of  the  behavior  of  magnesia 
and  bone-ash  cupels  during  cupellation  is  apparent.  It  will  be 
noted:  (1)  That  in  magnesia  cupels  the  lead  is  less  bright  and 
hence  at  a  lower  temperature  than  in  bone-ash  cupels,  although 
the  muffle  temperature  is  the  same.  This  is  due  to  the  fact  that 
the  extra  heat  generated  by  the  combustion  of  the  lead  is  dif- 
fused as  rapidly  as  generated  by  the  superior  diffusivity  of  the 
magnesia  cupel  and  hence  cannot  serve  to  raise  the  temperature 
of  the  lead,  as  is  the  case  in  the  bone-ash  cupel.  Hence  for 
the  same  "muffle  temperature"  the  actual  cupellation  tempera- 
ture of  the  lead  in  the  magnesia  cupels  is  50°  to  60°  C.  lower 
than  in  the  bone-ash  cupels.  To  this  fact  is  due  the  lower 
losses  of  precious  metal  in  magnesia  than  in  bone-ash  cupels. 
From  the  discussion  under  "  cupellation  temperature"  it  will  have 
been  noted  that  with  bone-ash  cupels,  if  once  the  muffle  has 
attained  a  temperature  sufficiently  high  to  cause  the  uncovering 
of  the  button,  the  rise  in  temperature  of  the  lead  due  to  its 
oxidation,  is  sufficient  to  carry  the  cupellation  to  a  finish  pro- 
vided the  muffle  temperature  is  not  lowered  at  the  end  of  the 

1  Bannister  and  Stanley,  "Thermal  Properties  of  Cupels."  Bui.  56.  I.  XL  M.  (1909). 


104 


A    MANUAL    OF   FIRE    ASSAYING 


operation.  This  is  not  the  case  with  magnesia  cupels  for  now 
obvious  reasons*  and  it  will  be  necessary  to  raise  the  muffle  tem- 
perature toward  the  end  of  the  operation  or  what  amounts  to  the 
same  thing,  push  the  cupel  to  the  hotter  part  of  the  muffle. 
Assayers  who  are  used  to  bone-ash  cupels,  therefore,  have  some 
difficulty  at  first  due  to  "freezing"  of  buttons  when  using 
magnesia  cupels. 

2.  Magnesia  cupels  retain  a  higher  temperature  longer  than 
bone-ash  cupels  when  withdrawn  from  the  furnace  or  moved  to 
the  cool  part  of  the  muffle,  and  hence  silver  buttons  show  a 
lesser  tendency  to  sprout,  due  to  the  slow  cooling  they  undergo. 

The  lead  in  magnesia  cupels  seems  to  open  somewhat  more 
readily  and  cupels  slightly  faster  than  in  bone-ash  cupels. 

The  accompanying  tables  give  data  of  results  obtained  by  bone- 
ash  and  magnesia  cupels  on  pure  silver  and  on  a  copper  matte.1 


TABLE   XVI.— COMPARISON  OF  BONE-ASH  AND  MAGNESIA 
CUPELS  ON  C.  P.  SILVER  CUPELLED  WITH  10  GRAMS 
SHEET  LEAD 


Amount  of  silver 
taken,  mgs. 

Bone-ash  cupels. 
Silver  bead,  weight,  mgs. 

Magnesia  cupels 
(Morganite)  . 
Silver  bead,  weight,  mgs. 

5 

4.85 

4.80 

10 

10.00 

10.00 

15 

14.72 

20 

19.30 

25 

24.41 

15 

14.36 

14.50 

20 

'      18.92 

19.52 

25 

23.84 

5 

4.94 

4.89 

10 

9.68 

9.86 

15 

14.70 

14.80 

20 

19.98 

19.68 

25 

24.60 

24.84 

The  sheet  lead  used  contained  a  little  silver.  Cupellation  in 
most  cases  was  carried  out  with  feathers.  It  is  to  be  noted  that 
when  low  finishing  temperatures  are  employed,  as  is  apt  to  be 
the  case  with  magnesia  cupels,  the  beads  may  retain  small 

1  By  O.  A.  Anderson  and  C.  H.  Fulton,   S.  D.  School  of  Mines,  Laboratory. 


CUPELLATION 


105 


amounts  of  other  metals  notably,  lead,1  to  which  may  be  due  in 
some  cases  the  higher  results  obtained. 

TABLE    XVII.— COMPARISON    OF     BONE-ASH    AND    MAGNESIA 
CUPELS  ON  A  COPPER  MATTE 


No. 

Weight 
lead 
button 
grams 

Matte 
taken 
a.  t. 

Bone-ash  cupels 

Magnesia  cupels(Morg.) 

Au  +  Ag 

Ag 

Au 

Au+Ag 

Ag 

Au 

1 
2 
3 

4 
5 
6 
7 
8 
9 
10 
11 

18 
24 
28 
30 
14 
18 
25 
10 
14 
31 
24 

0.05 
0.10 
0.15 
0.25 
0.05 
0.10 
0.20 
0.05 
0.10 
0.20 
0.25 

11.0 
21.12 
31.30 
56.10 
11.0 
21.8 
44.5 
10.9 
21.3 
42.6 
53.0 

10.38 
19.80 
29.22 
52.82 
10.36 
19.48 
41.90 
10.30 
20.10 
40.04 
49.75 

0.62 
1.32 
2.08 
3.28 
0.64 
1.32 
2.60 
0.60 
1.20 
2.56 
3.25 

13.0 
23.3 
27.2 

12.37 
22.02 
25.17 

0.63 
1.28 
2.03 

12.50 
27.0 
47.0 
11.2 
22.0 
43.1 
53.6 

11.82 
25.67 
44.37 
10.58 
20.70 
40.52 
50.28 

0.68 
1.33 
2.63 
0.62 
1.30 
2.58 
3.32 

The  assays  given  in  the  table  were  made  by  the  excess  litharge 
method.  The  average  result  stated  in  ounce  per  ton  is  as  follows: 
for  bone-ash  cupels,  gold  12.86  oz.,  silver  202.67  oz.;  for  magnesia 
cupels,  gold  13.16  oz.,  silver  222.76  oz.  These  results  are  un- 
corrected  assays,  viz.,  do  not  include  the  slag  or  cupel  absorption. 
In  practice  it  was  found  necessary  to  make  these  corrections  to 
obtain  concordant  results.  It  will  be  noted  that  the  magnesia 
cupels  give  higher  results  on  gold  and  very  much  higher  results 
on  silver.  This  last  is  without  question  due  in  large  part  to 
the  retention  of  copper  by  the  beads,  and  calls  for  caution  in  the 
use  of  magnesia  cupels  on  this  type  of  material. 

Portland  Cement  Cupels. — During  cupellation  Portland  cement 
cupels  act  very  similarly  to  bone-ash  cupels.  The  loss  is  some- 
what higher  than  in  bone-ash  cupels.  The  accompanying  table 
gives  losses  in  Portland  cement  cupels  and  bone-ash  cupels 
and  those  made  of  one-half  of  each  material.  The  temperatures 
are  average  temperatures  during  cupellation,  from  the  opening 
of  the  button  to  the  "blick."  One  hundred  mgs.  of  silver  were 

»  D.  M.  Liddell,  Eng.  and  Min.  Jour.,  LXXXIX.  254. 


106 


A   MANUAL    OF    FIRE    ASSAYING 


cupelled  with  about  20  grams  of  lead.1  The  temperatures  were 
measured  by  inserting  a  thermocouple  into  a  hole  bored  beneath 
the  bowl  of  the  cupel.  They  hence  represent  a  temperature  which 
is  a  mean  between  that  of  the  cupelling  lead  and  a  muffle  "blank" 
cupel. 

TABLE  XVIII.— CUPELLATION  LOSSES  WITH  DIFFERENT 
TYPES- OF  CUPELS. 


Average 

U.  S.  Portland 

R.  D.  Portland 

One-half  cement, 

Bone 

temp. 

cement, 

cement, 

one-half  bone-ash, 

ash,  loss 

deg.  C. 

loss  per  cent.    {    loss  per  cent. 

loss  per  cent. 

per  cent. 

915 

1.30 

1.34 

1.21 

1.26 

925 

1.81 

1.72 

1.54 

1.70 

945 

2.53 

2.56 

2.42 

2.42 

965 

3.37 

3.42 

3.05 

2.96 

Another  test  to  determine  the  relative  absorption  of  bone-ash 
and  cement  cupels2  gave  the  following  results:  On  10  mgs. 
silver  with  15  grams  lead,  at  an  orange  heat  (very  high)  cement 
cupels  showed  6.64  per  cent,  absorption  and  bone-ash  cupels, 
6.38  per  cent.  At  a  light  cherry  heat,  cement  cupels  showed 
4.91  per  cente  and  bone-ash  4.62  per  cent,  absorption.  It  is  to 
be  noted  that  the  percentage  absorption  other  factors  being  equal 
is  dependent  on  the  amount  of  precious  metal  cupelled  (see 
p.  163).  In  using  cement  cupels,  the  beads  must  be  carefully 
cleaned  otherwise  when  parting  in  nitric  acid  insoluble  silica 
is  apt  to  remain  which  will  be  weighed  as  gold.  The  bead  on 
cement  cupels  is  likely  to  be  more  flat  than  on  bone-ash  cupels. 

1  Holt  and  Christensen,  Eng.  and  Min.  Jour.,  XC,  560.     "Experiments  with  Portland 
Cement  Cupels." 

2  J.  W.  Merritt,  "Cement  vs.  Bone-ash  Cupels,"  Min.  and  Set.  Press.,  C,  649. 


CHAPTER  VIII 
PARTING 

Parting  is  the  separation  of  gold  from  silver  by  means  of 
acid.  In  assaying,  nitric  acid  is  almost  exclusively  used,  although 
sulphuric  acid  may  be  employed.  In  order  to  separate  silver 
from  gold  by  means  of  acid,  it  is  essential  that  there  be  present 
at  least  twice  as  much  silver  as  gold.  When  less  silver  is  present, 
it  is  impossible  to  separate  all  of  the  silver  from  gold  by  means 
of  acid  (see  assay  of  gold  bullion,  in  Chapter  XII).  When  the 
above-stated  amount  is  present,  it  requires  acid  of  not  less  than 
1.26  specific  gravity,  boiling  for  at  least  20  or  30  minutes,  to 
separate  the  silver  from  gold.  The  ratio  of  2  and  2.5  to  1  is 
used  practically  only  in  the  bullion  assay. 

In  parting  beads  from  ore  assays,  it  is  considered  necessary 
to  have  at  least  five  times  as  much  silver  as  gold  present.  The 
addition  of  silver  to  gold  or  to  the  gold-silver  alloy  in  order  to 
prepare  for  parting  is  termed  "inquartation,"  from  the  fact  that 
at  least  3  parts  of  silver  to  1  part  of  gold  were  formerly  con- 
sidered necessary.  The  nitric  acid  usted  for  parting  must  be  free 
from  hydrochloric  acid  and  chlorine  in  order  not  to  have  a  solvent 
action  on  the  gold.1  Nitric  acid  should  be  examined  for  chlorides 
before  being  used  for  parting.  In  order  to  part  silver  from  gold 
successfully,  the  following  points  must  receive  careful  considera- 
tion: (1)  The  strength  of  the  acid  used;  (2)  the  temperature  of 
the  acid;  (3)  the  ratio  of  gold  to  silver  in  the  bead  to  be  parted. 

1.  The  proper  strength  of  acid  is  of  great  importance.  For- 
merly, most  authorities  recommended  that  acids  of  1.16  and  1.26 
sp.  gr.  respectively — 2  parts  water  to  1  of  acid  (1.42  sp.  gr.) 
and  1  of  water  to  1  of  acid — be  used,  first  the  weak  acid  and  then 
the  stronger  acid.  T.  K.  Rose  recommends  4  parts  acid  to  3 
parts  water,  which  strength,  if  the  acid  be  heated,  will  not 
break  up  the  gold  in  the  bead  into  fine  particles,  even  if  50  parts  of 
silver  are  present  to  1  part  of  gold.  Gold  is  less  apt  to  break  up 
when  it  is  less  than  0.10  mg.  in  weight.  Keller2  recommends 

1  Consult  the  caption  "Solution  of  Gold  by  HNO3,"  in  Chapter  XI. 

2  Keller,  Trans.  A.  I.  M.  E.,  XXXVI,  3. 

107 


108  A    MANUAL    OP    FIRE    ASSAYING 

acid  of  the  following  strength:  1  part  acid  (sp.  gr.  1.42)  to  9  parts 
distilled  water.  In  this  strength  of  acid  the  gold  almost  invari- 
ably remains  in  a  coherent  mass,  even  when  the  silver  is  500 
times  as  much  as  the  gold.  This  is  the  strength  of  acid  recom- 
mended for  ordinary  assay  purposes.  The  beads  should  be  boiled 
in  the  acid  for  at  least  10  to  15  minutes  in  order  to  insure  parting. 

2.  It  is  essential  to  have  the  acid  at  the  boiling-point  before 
dropping  in  it  the  bead  to  be  parted.     Putting  the  bead  into  cold 
acid  and  heating  up  gradually  is  almost  certain  to  leave  the  gold, 
especially  where  the  ratio  of  silver  to  gold  is  high,  in  a  powdered, 
fine  condition,  very  apt  to  cause  losses  in  washing  and  subsequent 
handling  of  the  gold.     Cold  acid  should  not  be  used. 

3.  While  the  best  ratio  of  silver  to  gold,  for  parting  ordinary 
beads,  is  5  to  1,  this  ratio  is  not  always  under  control,  since  the 
assayer  must  be  content  in  many  cases  with  the  ratio  that  the 
ore  furnishes  him,  when  this  is  more  than  5  to  1.     If  less  than 
5  to  1,  silver  should  be  added  in  order  to  bring  it  up  to  this  ratio. 
The  silver  may  be  added  directly  to  the  crucible  or  scorification 
fusion,  or  to  the  lead  button  during  cupellation  if   it  is  not 
essential  to  determine  the  silver  in  the  ore. 

If  it  is  essential  to  determine  the  silver,  and  inquartation 
is  necessary,  the  bead  from  the  cupellation  is  first  weighed,  the 
requisite  amount  of  silver  is  added  to  the  bead,  both  wrapped  up 
in  about  2  grams  of  sheet  lead,  and  then  it  is  recupelled  and 
parted. 

Beads  which  need  inquartation  may  also  be  fused  with  silver, 
on  a  piece  of  charcoal,  by  means  of  the  blowpipe;  but  this  method 
is  not  to  be  recommended,  as  it  frequently  occasions  loss. 

Many  assayers,  if  they  suspect  an  ore  to  be  deficient  in 
silver  for  parting,  add  silver  to  the  crucible,  not  determining  the 
silver  in  this  assay,  but  running  a  separate  scorification  assay  for 
this  purpose. 

Another  way1  is  to  add  to  the  charge  a  desired  number  of  cubic 
centimeters  of  AgN03  solution  of  such  strength  that  1  c.c. 
contains  1  mg.  Ag.  The  proper  deduction  can  then  be  made  from 
the  weight  of  the  bead,  but  some  allowance  must  be  made  for 
the  silver  absorbed  by  the  cupel. 

After  parting,  the  acid  is  poured  from  the  parting  cup  or 
flask  in  which  the  operation  has  been  conducted,  and  the  gold 
residue  is  washed,  at  least  three  times,  with  warm  distilled  water 

1F.  G.  Hawley,  Eng.  and  Min.  Jour.,  XC,  649. 


PARTING 


109 


in  order  to  remove  all  trace  of  silver  nitrate.  The  black  stain 
occurring  in  parting  cups  after  heating  for  the  annealing  of  the 
gold  is  due  to  metallic  silver  reduced  from  silver  nitrate  by  the 


FIG.  56. — PARTING  FLASKS. 


heat,  showing  insufficient  washing.  Parting  may  be  carried  on 
in  small  porcelain  crucibles  called  "  parting  cups, "  or  in  test- 
tubes,  or  in  flasks  similar  to  copper-assay  flasks.  In  order  to 


110  A   MANUAL    OF   FIRE    ASSAYING 

part  in  flasks  or  test-tubes,  it  is  essential  to  have  the  gold  stay  as 
a  coherent  mass,  so  as  to  prevent  loss  in  transference.  When 
parting  cups  are  used,  after  washing,  the  gold  is  carefully  dried 
and  the  gold  annealed  at  a  dull-red  heat,  either  in  the  muffle  or 
by  means  of  the  blowpipe.  After  acid  treatment,  the  gold  is  left 
as  a  soft  black  mass,  probably  an  allotropic  condition  of  the  gold; 
but  upon  heating  this  is  changed  to  the  normal  yellow  metallic 
state  in  which  it  is  weighed.  Fig.  55  shows  a  convenient  parting 
bath  with  test-tubes;  Fig.  56  shows  parting  flasks  commonly  in 


CHAPTER  IX 
THE  ASSAY  OF  ORES  CONTAINING  IMPURITIES 

Impurities,  from  the  assayer's  point  of  view,  are  such  sub- 
stances, contained  in  ores,  furnace  products,  or  other  material, 
as  necessitate  some  particular  method  of  assay  or  treatment,  or 
the  observing  of  special  precautions  not  included  in  the  ordinary 
crucible  assay  as  already  outlined. 

Common  impurities  are  sulphur,  arsenic,  tellurium,  antimony, 
zinc,  copper,  etc.  Of  these  sulphur  is  by  far  the  most  common. 

In  performing  an  assay  it  is  usually  the  aim  of  the  assayer, 
whenever  this  is  possible,  to  produce  by  direct  fusion,  either  by 
the  crucible  or  scorification  method,  a  pure  lead  button  weighing 
approximately  20  grams.  If  the  button  is  smaller  than  this, 
there  is  danger  of  not  collecting  the  values;  if  larger,  cupellation 
is  too  prolonged  and  losses  are  increased.  In  the  assay  of  low- 
grade  gold  ores  it  may  be  desirable  to  produce  lead  buttons  of 
25  to  30  grams  in  order  to  obtain  the  best  results.  The  impurities 
mentioned  affect  either  the  size  of  the  button,  or  the  purity  of  the 
button,  or  both.  To  show  the  effect  of  sulphur  the  following 
definite  example  is  taken. 

Given  an  ore  containing  pyrite,  which,  in  a  charge  yielding 
the  ordinary  type  of  monosilicate  slag,  gives  a  reducing  power 
of  5  grams  of  lead  per  gram  of  ore.  If  the  following  charge, 

15  grams  of  ore  70  grams  of  PbO 

30  grams  of  Na2CO,  8  grams  of  SiO2 

Borax  glass  cover 

be  made  up  and  fused,  a  60-gram  button  (approximately)  will  be 
produced,  on  top  of  which  will  be  a  small  quantity  of  "  matte, " 
i.e.,  an  artificial  sulphide  of  the  metals,  in  this  case  iron  and  lead. 
This  matte  is  brittle  and  may  contain  some  silver  and  a  little  gold. 
On  hammering  the  button,  it  is  lost.  In  general,  it  is  an  undesir- 
able product  to  make.  A  small  amount  of  matte  is  produced  in 
this  case,  since  the  ore  has  the  power  to  reduce  75  grams  of  lead 
from  PbO,  while  only  70  grams  of  PbO  are  present,  so  that  the 

111 


112  A    MANUAL    OF    FIRE    ASSAYING 

excess  sulphide  of  the  ore  not  acted  upon  by  the  PbO  remains  in 
the  charge,  uniting  with  some  of  the  lead  to  form«a  sulphide  of 
iron  and  lead.  The  button  is  also  much  too  large  to  cupel.  If  in 
the  charge  the  PbO  is  materially  increased,  the  ore  will  react  to 
the  extent  of  its  full  reducing  power,  a  lead  button  of  75  grams 
will  be  produced,  no  matte  will  be  found,  and  the  slag  will  be 
improved,  owing  to  the  addition  to  it  of  the  fusible  base  PbO. 
If  the  PbO  in  the  charge  be  materially  reduced,  the  lead  button 
will  be  much  smaller  (owing  to  the  dearth  of  PbO  available  for 
reduction),  considerable  matte  will  be  formed,  and  the  slag  will 
be  poor. 

If  the  silica  be  increased,  so  that  sufficient  be  present  to  form 
the  higher  silicates  with  all  the  bases  present,  practically  no 
lead  will  be  reduced,  for  the  sulphide  has  not  the  power  to  reduce 
miich  Pb  from  lead  soda  silicates  unless  a  free  base  be  present,  e.g., 

(PbO.Na2O)2SiO2+FeS2  (no  action), 
or  possibly 

(PbO.Na20)2Si02  +FeS2  =  (FeO.Na2O)2SlO2  +PbS  +  S. 

In  this  way  the  sulphur  remains  in  the  charge  in  the  form  of 
sulphide  sulphur. 

Soda  will  cause  the  formation  of  SO3,  if  PbO  is  present  to 
furnish  the  oxygen,  and  if  it  can  act  as  a  free  base,  i.e.,  if  it  is 
not  combined  with  silica  (see  Chapter  V,  on  Reduction  and 
Oxidation  Reactions) .  An  increase  of  soda  without  an  increase 
of  PbO  or  SiO2  will  lessen  the  amount  of  matte,  as  sulphur  will 
tend  to  combine  to  some  extent  with  the  Na20  to  form,  writh  the 
FeS,  a  double  sulphide  of  iron  and  soda,. etc.,  which  will  be  dis- 
solved in  the  slag.  The  above  outlines  the  effect  of  such  impur- 
ities as  sulphur  and  arsenic,  and  shows  the  necessity  of  special 
methods  of  assay  directed  toward  the  getting  rid  of  impurities. 

The  impurities  mentioned  may  be  divided  into  two  classes: 

(a)  Those  which  can  be  volatilized  by  oxidation  or  otherwise, 
e.g.,  sulphur,  arsenic,  and  antimony! 

(6)  Those  which  cannot  be  volatilized,  e.g.,  copper,  zinc,  etc. 

Some  of  these  may  be  partly  volatilized,  as  antimony  and 
zinc.  For  the  removal  of  all  of  them,  however,  whether  by 
volatilization  or  by  slagging,  oxidation  is  essential. 

In  one  method  employed  on  light  sulphide  or  arsenic  ores, 
the  iron-nail  method,  sulphur  and  arsenic  are  carried  into  the 
slag  as  a  double  sulphide  or  arsenide  of  soda  and  iron,  etc. 


Crucible  Fusions. 


ASSAY    OF    ORES    CONTAINING    IMPURITIES  113 

The  following  methods  are  standard  methods  for  the  assay 
of  impure  ores,  and  are  discussed  in  detail: 

1.  The  roasting  method. 

2.  The  niter  method. 

(a)  The  common  niter  method. 
(6)  Miller's  oxide  slag  method, 
(c)  Perkins'  excess-litharge  method. 

3.  The  iron-nail  method. 

(a)  The  niter-iron  method. 

4.  The  cyanide  method  (rarely  used). 
*5.  .The  scorification  method. 

6.  The    combination   wet-and-dry   method 
(removal  of  impurities  by  solution) . 

THE  ROASTING  METHOD.— It  is  usual  to  carefully  weigh  out 
0.5  or  1  assay  ton  of  the  ore  to  be  assayed,  and  place  it  in  a  roast- 
ing dish  of  sufficient  size  to  permit  of  stirring  without  loss  by 
spilling.  The  dish  is  placed  in  the  muffle,  the  temperature  of 
which  is  not  above  a  "  black  red  "  and  the  firing  of  which  is  under 
good  control,  so  that  the  temperature  will  not  rise  too  rapidly. 
In  the  case  of  an  ordinary  sulphide  ore,  such  as  a  pyrite,  or,  for 
example,  a  chalcopyrite  and  quartz,  the  following  reactions  take 
place,  if  the  roasting  is  carried  on  slowly  at  a  low  heat:1 
3CuFeS2  +  ISO  +  heat  =  Cu2S  +  3FeSO4  +  CuSO4  +  SO2 

At  590°  C.  the  ferrous  sulphate  decomposes  spontaneously, 
sulphatizing  the  balance  of  the  copper: 

Cu2S  +  2FeSO4+6O  =  2CuSO4+Fe2O3  +  SO3 

At  655°  C.  the  copper  sulphate  decomposes 'into  basic  sulphate 
and  SO3,  and  at  700°  C.  into  CuO  and  SO3,  as  follows: 
2CuSO4  =  CuO.CuSO4  +  S03, 
CuO.CuS04  =  2CuO+S03; 

so  that  the  final  products  of  the  roast,  when  carried  to  above 
700°  C.,  are  ferric  and  cupric  oxide,  with  a  complete  removal  of 
the  sulphur.  If  the  temperature  is  not  carried  above  700°  C., 
sulphur  remains  in  the  charge  as  sulphate,  which  may  again  be 
reduced  in  the  crucible  to  sulphides: 

2CuSO4  +  3C  =  Cu2S  +  S02  +  3CO2 

If,  for  any  reason,  it  is  not  desirable  to  carry  the  temperature 
as  high  as  700°  C.,  the  ore,  after  roasting  until  no  further  smell 

1  R.  H.  Bradford,  Trans.  A.  I.  M.  E.,  XXXIII,  68. 


114  A   MANUAL    OF    FIRE    ASSAYING 

of  SO2  is  discernible,  is  cooled  and  mixed  with  5  to  10  grams  of 
powdered  (NH4)  2CO3,  and  reroasted  at  a  low  heat,  the  sulphuric 
anhydride  (SO3)  being  eliminated  as  volatile  ammonium.sulphate, 
(NH4)2S04: 

CuS04  +  (NH4)  2C03  =  CuO  +  (NH4)  2SO4  +  C02 

Any  silver  in  the  ore  that  has  been  roasted  will  be  in  the  form 
of  Ag2SO4,  or  if  arsenic  and  antimony  are  present,  partly  in  the 
form  of  arseniates  and  antimoniates.  If  the  roasting  temperature 
is  carried  to  870°  C.  and  above,  the  silver  sulphate  will  be  de- 
composed, leaving  the  silver  in  the  form  of  metallic  silver.  In 
order  to  avoid  loss  of  silver  it  is  best  not  to  carry  the  temperature 
above  700°  C. 

In  roasting  simple  pyrite  ores,  the  reactions  are  similar,  but 
simpler,  and  the  temperature  need  not  be  carried  above  600°  C. 
During  roasting,  the  ore  should  be  stirred  frequently  in  order  to 
expose  fresh  surfaces  to  oxidation. 

When  ores  contain  arsenic  and  antimony,  the  roasting  opera- 
tion is  more  difficult  and  complex,  and  considerable  care  and 
skill  are  required  to  eliminate  the  greater  part  of  these  two  volatile 
elements.  The  reason  for  this  is  that  the  arsenic  and  antimony 
pass  by  roasting  first  to  the  state  of  the  lower  oxides  As203, 
Sb2O3,  which  are  volatile,  and  then  to  the  state  of  the  higher 
oxides  As2O5,  Sb2O5,  forming  arseniates  and  antimoniates  of  cer- 
tain metals  present  in  the  ore,  some  of  which  are  stable  even  at 
high  temperatures,  thus  fixing  the  arsenic  and  antimony  in  the 
roasted  ore,  and  not  eliminating  it.  The  arseniates  (or  anti- 
moniates) which  ordinarily  form  are  those  of  copper,  iron  and 
silver.  The  best  conditions  for  the  elimination  of  arsenic  and 
antimony  are  alternate  oxidation  and  reduction  at  a  low  heat. 
The  presence  of  sulphur  tends  to  aid  the  elimination  of  arsenic 
and  antimony  by  the  formation  of  the  volatile  sulphides  of  these 
elements.  The  reducing  action  necessary  for  the  elimination  of 
arsenic  and  antimony  is  best  obtained  by  mixing  wTith  the  ore 
equal  volumes  of  coal  dust  or  charcoal,  and  roasting  at  a  dark  red 
heat  until  the  coal  is  burnt  off,  then  cooling,  adding  more  coal 
dust,  and  reroasting.  In  this  way  the  greater  part  of  the  arsenic 
and  antimony  can  be  readily  volatilized,  except  in  very  rich 
silver  ores.  When  galena  ores  are  to  be  roasted,  the  ore  is  best 
mixed  with  an  equal  volume  of  silica  and  roasted  at  a  very  low 
heat.  In  this  roast  PbSO4  is  formed  to  a  considerable  extent, 


ASSAY    OF    ORES    CONTAINING    IMPURITIES  115 

which  at  a  higher  heat  is  decomposed  by  the  SiO3  present,  as 
follows: 

PbSO4  +  SiO2 = PbSiO3  +  SO3 

Care  must  be  taken  with  this  roast  as,  at  the  formation  point 
of  lead  silicate,  silver  losses  are  apt  to  occur.  A  successful  roast 
will  be  indicated  by  a  yellow  color  (lead  silicate),  and  an  unsuc- 
cessful one  by  a  black  or  gray  color  (fused,  undecomposed  sul- 
phides). In  general,  heavy  sulphide  ores  that  contain  their  chief 
value  in  gold  may  be  roasted,  when  this  is  carefully  done,  without 
loss  of  gold;  but  silver  ores,  especially  when  of  high  grade,  are 
apt  to  give  low  results. 

In  making  up  the  charge  for  the  roasted  ore,  it  is  to  be  noted 
that  from  a  sulphide  ore  (pyrite,  etc.)  the  product  is  frequently 
of  an  oxidizing  nature  and  basic,  which  must  be  taken  into  account 
in  adding  the  fluxes.  In  galena  ores,  when  silica  has  been  added, 
this  must  be  accounted  for. 

The  roasting  method  is  frequently  used  for  heavy  sulphide 
ores,  especially  when  they  have  a  low  value  in  gold  and  silver, 
as  it  permits  of  a  large  amount  of  ore  being  taken  (1  assay  ton 
and  more),  which  after  roasting  presents  no  difficulty  in  making 
the  proper  fusion. 

THE  NITER  METHOD.— The  first  step  in  the  niter  method  is  the 
making  of  a  preliminary  assay  according  to  the  directions  already 
given.  The  precautions  concerning  the  reducing  power  of  the 
sulphides  in  different  types  of  charges  must  be  carefully  noted; 
it  is  best  to  have  the  preliminary  charge  of  the  same  composition 
as  the  final  assay  charge.  Or  else  the  reducing  power  may  be 
determined  by  the  soda-litharge  charge  and  this  cut  down  by 
25  per  cent.,  20  grams  deducted  for  the  lead  button,  and  the 
remainder  divided  by  4  to  get  the  amount  of  niter  to  add,  in 
grams,  if  the  monosilicate  slag  is  to  be  made  in  the  assay. 

The  amount  of  ore  taken  for  the  niter  assay  varies  according  to 
the  grade  of  the  ore  in  gold  and  silver  and  according  to  the  amount 
of  impurity  present.  It  is  rarely  desirable  to  add  more  than 
20  grams  of  niter  to  the  charge,  as  larger  amounts  cause  difficulty 
through  the  evolution  of  too  much  gas.  One-half  assay  ton 
is  the  amount  of  ore  most  frequently  taken.  Sometimes, 
with  ores  containing  much  impurity,  0.10  to  0.25  assay  ton  is 
used.  Twenty-gram  crucibles  (170  c.c.  capacity)  are  used  for 
amounts  of  0.5  assay  ton  of  ore  and  less,  and  30-gram  crucibles 
(240  c.c.  capacity)  for  1  assay  ton  of  ore. 


116  A    MANUAL    OF    FIRE    ASSAYING 

MILLER'S  OXIDE-SLAG  METHOD. — This  method  is  a  modified 
niter  method  applicable  to  such  ores  as  contain  practically  no  silica; 
i.e.,  heavy  sulphide  ores,  such  as  pyrites,  arsenopyrite,  mattes, 
etc.  It  is  based  on  the  fact  that  PbO  has  the  power  to  hold  in 
solution  and  in  suspension  oxides  of  such  metals  as  copper,  iron, 
etc.  (see  p.  122,  where  " scorification "  is  discussed),  in  certain 
amounts.  Niter  is  added  to  oxidize  the  sulphides,  etc.,  and 
Na2CO3  to  aid  in  the  complete  oxidation  of  the  sulphur  by  the 
formation  of  sulphates,  in  the  manner  already  discussed.  The 
first  step,  as  in  the  ordinary  niter  method,  is  the  preliminary 
assay,  according  to  the  following  charge: 

Ore 3  grams 

PbO 50  grams 

Na2CO3 8  grams 

The  final  charge  is  as  follows: 

Ore 0.5  assay  ton 

PbO 70.0  grams 

Na2CO3 12.0  grams 

KNO3 (calcuated  for  a  20-gram  button) 

Quick  fires,  1100°  C.,  30  minutes,  are  found  to  be  best.  The 
slags  are  usually  dull  black  and  pour  readily,  and  the  button 
separates  easily  from  the  slag.  (In  slags  high  in  silica  or  con- 
taining much  borax,  the  lead  buttons  are  apt  to  adhere  closely 
to  the  slag.)  With  the  oxide-slag  method,  trouble  is  sometimes 
experienced  through  the  lead  refusing  to  collect  and  remaining 
shotted  through  the  slag.  The  difficulty  is  usually  due  to  too 
much  soda  (especially  if  considerable  niter  is  used)  although  too 
low  a  temperature  of  fusion  is  also  a  factor. 

The  method  gives  reliable  results  on  gold  and  silver,  compar- 
ing well  with  the  other  standard  methods.1 

PERKINS'  EXCESS-LITHARGE  METHOD. 2— This  method  is  based 
on  the  fact  that  PbO  will  dissolve  oxides  of  other  metals  and,  if 
present  in  great  excess,  will  prevent,  to  a  large  extent,  the  reduc- 
tion of  other  metals,  such  as  Cu  and  Sb.  The  presence  of  so 
much  PbO  also  insures  a  strongly  oxidizing  tendency  in  the 
crucible,  preventing  impurities  entering  into  the  button. 

It  is  desirable  to  add  or  have  present  SiO2  in  such  an  amount 
as  will  form  a  monosilicate  with  the  bases  present,  including 

1  Miller,  Hall   and  Falk,   "The  Reduction  of  Lead  from  Litharge,"  etc.,  in  Trans.  A.  I. 
M.  E.,  XXXIV,  398.  399. 
*  W.  G.  Perkins,  "The  Litharge  Process,"  ibid.,  XXXI,  913. 


ASSAY    OF    ORES    CONTAINING    IMPURITIES  117 

some  litharge,  but  leaving  much  litharge  uncombined  in  the 
"charge. 

The  following  table  shows  the  proportion  of  PbO  required  to 
form  fusible  compounds  with  the  principal  metallic  oxides:1 

TABLE  XIX.— PbO  REQUIRED  WITH  METALLIC  OXIDES 

One  part  of Cu2O     CuO  ZnO    Fe3O«      FesO*      MnO    SnO2    SbzOi     As2Os 

Requires  parts  of  PbO..      1.5       1.8       84  10  10         13  5  1 

In  order  to  carry  out  the  excess-litharge  method  intelligently, 
it  is  necessary  to  know  the  approximate  composition  of  the  ore, 
so  as  to  provide  the  proper  amount  of  PbO  and  SiO2.  The  best 
fusion  exhibits,  in  a  section  of  the  cone  of  the  slag  after  breaking, 
silicates  of  lead,  iron,  etc.,  on  the  outer  surface,  gradually  passing 
to  crystalline  litharge  toward  the  center.  The  temperature  of 
fusion  should  not  exceed  1050°  to  1100°  C.  It  must  be  above 
884°  C.  (melting-point  of  PbO) .  The  first  step  is  the  making  of  a 
preliminary  assay  in  order  to  determine  the  amount  of  niter  to 
be  added.2 

The  final  charge  most  frequently  used  is: 

Ore. 0.25  to    0.5  assay  ton  Na2CO3 12  grams 

PbO 8        to  10      assay  tons  SiO2 10  grams 

Niter  to  obtain  20-gram  button 

The  button  is  generally  clean,  and  separates  easily  from  the 
slag. 

The  excess  litharge  method  will  give  somewhat  low  results  on 
silver,  especially  on  high  grade  ores  but  will  give  good  results  on 
gold.  In  ores  of  the  following  analysis,  SiO2  40  to  60%; 
Fe,  5%;  CaO,  2%;  Pb,  15  to 40%;  Zn,  2%;  Ag,  20 to 80  oz.;  S,  1%; 
and  a  trace  of  copper,  the  results  in  the  accompanying  table 
were  obtained  by  the  use  of  charges  A,  B  and  C.3 

Charge  A  Charge  B  Charge  C 


PbO  
Borax  glass.  .  . 
Flour 

25        grams 
4        grams 
2  .  25  grams 

50        grams 
4        grams 
2  .  25  grams 

75        grams 
4        grams 
2  .  25  grams 

NaHCO3  
K,CO»  
Ore.  .  . 

25  .  0    grams 
25  .  0    grams 
0.5    a.t. 

25  .  0    grams 
25  .  0  grams 
0.5    a.t. 

25  .  0    grams 
25  .  0    grams 
0.5    a.t. 

1  Hofman,  "Metallurgy  of  Lead,"  p.  7. 

2  In  place  of  niter,  it  may  be  necessary,  in  this  method  or  in  Miller's  method,  to  add  argol, 
if  ore  is  not  reducing. 

3  Kenneth  Williams,  Jour.  Ind.  and  Eng.  Chem.,  II,  406. 


118 


A    MANUAL    OF    FIRE    ASSAYING 


TABLE   XX.— AVERAGE   RESULTS    SHOWING    EFFECT    OF    AN 
INCREASE  OF  PuO  ON  SILVER  RESULTS 


Ore  No. 

Charge  A, 
ounces  Ag  per  ton 

Charge  B, 
ounces  Ag  per  ton 

Charge  C, 
ounces  Ag  per  ton 

1 

51.05 

50.86 

50.62 

2 

42.36 

42.20 

42.02 

3 

27.82 

27.70 

27.55 

An  ore  from  Cobalt,  Canada,1  containing  5.06  per  cent.  Ni 
and  9.12  Co,  chiefly  as  niccolite  and  smaltite,  and  some  free 
silver  was  assayed  by  the  following  charge: 

Ore 

NaHCO3 

Borax  glass 

Argol 


0.05  a.  t. 
10        grams 
10        grams 

1 . 5    grams 


Litharge as  given  in  table. 

TABLE  XXL— AVERAGE   RESULTS  SHOWING  THE   EFFECT   OF 
AN  INCREASED  AMOUNT  OF  PnO  ON  SILVER  RESULTS 


Litharge, 
grams 

Lead  button, 
grams 

Silver, 
ounces  per 
ton 

Silver  in  slag, 
ounces  per 
ton 

Silver  in  cupel, 
ounces  per 
ton 

30 

19 

2051.4 

9.6 

34.0 

40 

21 

2056.0 

40 

21 

2050  0 

80 

30 

1968.6 

80 

22 

1944.6 

135.2 

35.0 

80 

21 

1984.8 

70.2 

34.6 

80 

21 

1914.8 

THE  IRON-NAIL  METHOD.— This  method  does  not  attempt 
to  oxidize  impurities,  but  aims  to  carry  sulphur,  etc.,  into  the  slag. 
The  ore  is  decomposed  by  the  iron  nails  added  to  the  charge  and 
by  the  PbO  present.  As  iron  reduces  PbO  to  Pb,  the  amount  of 
litharge  added  to  the  charge  is  limited  to  25  to  30  grams.  The 
amount  of  soda  needed  is  large,  as  this  flux  is  depended  upon  to 
carry  the  sulphur  into  the  slag.  The  slag  should  be  below  a 
monosilicate  in  degree,  and  high  in  soda,  as  basic  alkaline  slags 
have  a  high  solvent  power  for  sulphides. 

1  R.  W.  Lodge,  "The  Effect  of  High  Litharge  in  the  Crucible  Assay  for  Silver,"  Trans. 
A.  I.  M.  E.,  XXXVIII.  638. 


ASSAY    OF    OKES    CONTAINING    IMPURITIES 


119 


A  typical  charge  on  an  ore  that  has  a  reducing  power  of  about 
4  grams  of  Pb  per  gram  of  ore  is  :l 

Ore 0.5  assay  ton         SiO2 2  grams 

NaHCO3 30      grams               borax 8  grams 

PbO 30      grams               nails 17  grams 

Salt  cover 

The  soda  should  usually  be  twice  the  amount  of  ore  in  the 
charge.     The  reactions  that  take  place  are  approximately  as 
follows: 
7PbO  +  FeS2  +  4NaHCO3  =  7Pb  +  2Na2SO4  +  FeO  +  4CO2  +  2H2O 

Part  of  the  ore  is  decomposed  by  the  PbO,  and  part  of  the  S 
may  go  off  as  SO2,  as  discussed  in  previous  pages.  The  iron 
nails  decompose  the  balance  of  the  sulphides: 


PbS+Fe  =  Pb+FeS  (if  galena  is  present  or  lead  sulphide  forms). 

The  iron  sulphide  (FeS)  is  dissolved  by  the  alkaline  slag, 
forming  probably  double  sulphides  of  soda  and  iron. 

To  show  the  nature  of  the  iron-nail  fusion,  the  following 
results  of  two  fusions  on  a  pyrite  ore  containing  39.5  per  cent. 
S — a  reducing  power  equal  to  about  8 — are  given:2 

Charge  1  Charge  2 

1  assay  ton ore 0.5  assay  ton 

30  grams NaHCO3 30  grams 

30  grams PbO 30  grams 

4  grams SiO2 4  grams 

4 nails 4.... 

10  grams borax  glass  cover.   10  grams 

The  following  results  were  obtained: 

No.  1  No.  2 

Slag 60  grams       65        grams 

Matte 23.5        grams     none 

Lead 24 . 5        grams       26 . 5    grams 

Crucible  and  charge  before  fusion 685  grams     662        grams 

Crucible  and  charge  after  fusion 665  grams     642        grams 

Loss  in  weight 20  grams       20        grams 

Nails  before  fusion 64  grams       63        grams 

Nails  after  fusion 43  grams       49        grams 

Loss  of  iron 21  grams       14        grams 

Per  cent,  of  S  in  slag 6.73  7.63 

Sin  slag 4.03      grams         4. 96  grams 

S  in  ore 1 1 . 85      grams         5 . 92  grams 

S  passed  off  as  S02 0.95      grams         0.96  grams 

S  in  matte 6 . 87      grams     none 

1  Lodge,  "Notes  on  Assaying,"  p.  99. 
3  Lodge,  iiid.,  p.  101. 


120  A   MANUAL    OF    FIRE   ASSAYING 

It  will  be  noted  that  the  charges  are  identical  as  far  as  the 
fluxes  are  concerned,  but  that  the  amount  of  ore  differs.  It  is 
desirable  in  heavy  sulphide  ores  to  keep  the  ore  down  to  0.5 
assay  ton  and  lower  if  necessary. 

Care  must  be  taken  not  to  have  the  slag  above  a  monosilicate 
in  degree,  for  if  higher  in  SiO2  there  will  be  particular  danger  in 
this  charge  of  not  having  the  sulphides  oxidized  by  the  PbO, 
more  sulphide  being  retained  in  the  charge  than  it  can  dissolve, 
and  forming  a  matte,  even  with  small  amount  of  ore. 

THE  NITER-IRON  METHOD. — This  method  is  in  principle  the 
same  as  the  iron-nail  method.  An  amount  of  niter  is  added  at 
random,  sufficient  to  oxidize  but  a  portion  of  the  sulphides,  the 
Balance  being  decomposed  by  the  nails. 

THE  CYANIDE  METHOD. — Sometimes,  when  no  other  fluxes  are 
at  hand,  or  when  a  rapid  assay  is  to  be  made  in  which  accuracy 
is  not  essential,  a  fusion  of  ore  with  cyanide  may  be  made,  and 
the  resultant  button  cupelled  for  silver  and  gold.  The  method 
is  a  rapid  one  and  gives  good  malleable  buttons,  but  is  apt  to  be 
low  in  gold  and  silver,  especially  in  silver.  The  cyanide  used 
should  be  pure,  free  from  carbonates  or  other  impurities,  and 
the  fusion  should  be  made  at  a  low  temperature.  The  following 
charge  is  used: 

Ore 0.5tol  assay  ton 

PbO 25  grams 

KCN 3  assay  tons 

When  the  ore  contains  copper  and  other  base-metal  impuri- 
ties, these  are  reduced  and  enter  the  lead  button.  Sulphur  is 
taken  up  by  the  slag  as  potassium-sulpho-cyanate  (KCNS).  In 
general,  it  is  a  method  not  to  be  recommended.  The  following 
results  show  the  loss  in  silver  which  takes  place  in  this  method.1 

TABLE  XXII.— LOSS  OF  SILVER  IN  CYANIDE  METHOD 


Niter  method 

Cyanide  method 

Silver,  by  uncorrected  assay  
Silver  in  slag  
Silver  from  cupel 

563  .  73  mgs. 
4.10  mgs. 
7.81  mgs. 

525  .  5    mgs. 
36  .  8    mgs. 
6  .  56  mgs. 

»  E.  H.  Miller.  "Corrected  Assays."  in  Seh.  Mines  Quart.,  XIX,  November.  1897. 


ASSAY    OF    ORES    CONTAINING   IMPURITIES  121 

The  results  are  averages  of  duplicate  assays.  The  loss  of 
gold  in  the  slag  by  cyanide  fusion  is  not  nearly  so  marked  as  that 
of  silver. 

A  COMPARISON  OF  THE  DIFFERENT  CRUCIBLE  METHODS 
OF  ASSAY  FOR  IMPURE  ORES. — In  very  impure  ores,  containing 
large  amounts  of  sulphur,  arsenic,  etc.,  the  roasting  method  is 
applicable  when  gold  only  is  to  be  determined,  or  when  silver 
results  need  not  be  very  accurate.  The  roasting  method  gives 
uniformly  lower  silver  results  than  most  of  the  other  methods, 
although  to  a  large  extent  this  is  due  to  roasting  at  too  high  a 
temperature.  The  roasting  method  has  the  advantage  that  when 
ores  are  low  grade  large  quantities  of  ore  can  be  taken,  which 
is  not  always  possible  with  the  other  methods.  Roasting,  however, 
must  be  skillfully  conducted  in  order  to  be  successful. 

The  niter  method  is  a  desirable  and  clean  method  of  assay 
giving  accurate  results.  Where  large  quantities  of  niter  are  em- 
ployed, the  oxidizing  action  in  the  crucible  is  greatly  increased, 
and  it  is  probable  that  thereby  losses  in  silver  are  apt  to  occur 
by  the  slagging  of  the  silver. 

There  is  no  accumulated  evidence  on  this  subject,  but  many 
assayers  hold  this  opinion.1  The  niter  method  is  desirable  for 
such  ores  as  do  not  contain  amounts  of  sulphur  requiring  extra- 
ordinary amounts  of  niter.  Usually,  the  limit  of  niter  in  a 
charge  is  placed  at  about  20  grams;  if  the  ore  should  require  more 
than  this,  it  is  generally  considered  advisable  to  reduce  the  quan- 
tity of  ore  taken  for  the  assay.  This  has  the  disadvantage  of 
multiplying  the  error  of  the  assay,  when  finding  the  value  per  ton. 

The  modified  niter  methods  discussed  offer  advantages  in  the 
slagging  of  base-metal  impurities.  This  is  particularly  true  of 
copper  and  zinc.  It  is- very  much  easier  to  cause  copper  to  enter 
the  slag  when  an  oxide  slag  is  made  than  when  a  silicate  is  made. 
This  is  partly  due  to  the  oxidizing  nature  of  the  high  litharge 
charges.  The  best  method  for  the  slagging  of  base-metal  impur- 
ities is  the  excess-litharge  method. 

The  iron-nail  method  is  a  standard  method,  which  can  be 
successfully  applied  to  most  sulphide  ores  and,  with  care,  to 
arsenical  ores.  It  is  not  applicable  to  ores  containing  base- 
metal  impurities,  such  as  copper,  for,  being  essentially  reducing 
in  its  nature,  practically  all  of  the  base-metal  impurities  will 
be  found  in  the  lead  button.  When  used  with  arsenical  ores, 

1  E.  C.  Woodward,  Minn,  and  Sri.  Press,  CII,  301,  gives  data  which  rather  tends  to 
show  that  niter  does  not  have  this  effect. 


122  A   MANUAL    OF   FIRE   ASSAYING 

the  temperature  employed  should  be  low,  not  above  1050°  C.; 
otherwise  *speiss  (an  artificial  arsenide  of  iron)  is  apt  to  form, 
which  may  carry  values.  It  also  has  the  objection,  in  the  case 
of  very  impure  ores,  that  small  quantities  must  be  taken  for 
assay,  involving  serious  risk  of  multiplying  an  error  of  assay. 

SCORIFICATION. — This  is  the  oxidizing  fusion  of  ore  with 
metallic  lead  in  the  muffle-furnace,  producing,  in  the  main,  a 
litharge  slag,  i.e.,  an  oxide  slag.  It  is  a  method  of  assay  which 
requires  no  previous  preparation  of  the  ore  or  preliminary  assay, 
and  as  practically  only  one  flux  is  employed,  it  is  both  a  cheap  and 
a  rapid  method.  It  is  also  a  thoroughly  reliable  method,  when 
proper  precautions  are  taken  and  when  it  is  employed  on  ma- 
terial suitable  for  the  purpose.  The  operation  is  performed  in 
shallow  fire-clay  dishes,  called  scorifiers. 

The  sizes  commonly  used  are: 

1 . 5-in.  scorifiers;  cubic  contents 15  c.c. 

2.0-in.  scorifiers;  cubic  contents 25  c.c. 

2. 5-in.  scorifiers;  cubic  contents 37  c.c. 

3 . 5-in.  scorifiers;  cubic  contents. . . . '. 100  c.c. 

The  dimensions  referred  to  are  outside  dimensions.  The 
size  most  commonly  employed  is  the  2.5-in.  one.  Before 
these  dishes  are  used  it  is  usual  to  line  the  inside  with  ferric 
oxide.  This  is  done  by  preparing  crushed  iron  ore  or  ochre, 
mixing  with  water,  and  painting  the  inside  of  the  dishes.  This 
gives  them  a  basic  lining,  and  to  some  extent  prevents  the  oxide 
slag  from  attacking  the  silica  in  the  clay. 

Some  scorifier  slags,  especially  if  they  contain  copper,  are 
very  corrosive.  The  amount  of  ore  taken  for  scorification 
varies  from  0.10  assay  ton  to  0.25  assay  ton;  but  0.10  assay 
ton  is  the  amount  most  frequently  taken.  The  larger  amounts 
are  rarely  used,  unless  the  ore  contains  practically  no  bases. 
Sometimes,  for  very  impure  material,  as  little  as  0.05  assay 
ton  is  taken.  The  amount  of  test  lead  varies  according  to  the 
nature  of  the  ore.  The  more  impure  the  ore  the  larger  will  be 
the  ratio  of  lead  to  ore.  With  0. 10  assay  ton  the  test  lead  will 
vary  from  40  to  100  grams.  A  common  charge  is  40  to  50 
grams  of  test  lead  for  ordinary  ores.  As  already  pointed  out, 
certain  quantities  of  litharge  are  required  in  order  to  make 
fusible  compounds  with  the  metallic  oxides.  If  the  ore  con- 
tains small  amounts  of  the  metallic  oxide,  the  test  lead  will  be 


ASSAY    OF    ORES    CONTAINING    IMPURITIES  123 

small  in  amount;  if  it  contains  appreciable  quantities  of  ferric 
oxide  (Fe203)  or  Cu,  etc.,  large  amounts  of  test  lead  will  be  re- 
quired. It  is  best  to  add  a  small  amount  of  borax  glass  to  the 
charge,  from  1  to  1.5  grams,  scattering  it  over  the  surface  of 
the  lead.  This  aids  in  the  solution  of  the  bases  present.  When 
the  ore  contains  the  basic  oxides  mentioned,  borax  glass  up 
to  3  and  4  grams  will  materially  aid  in  forming  good  slags, 
without  infusible  scoria.  This  infusible  scoria  often  appears 
in  ores  containing  large  amounts  of  bases,  and  is  very  apt  to 
give  low  results  by  entangling  unfused  portions  of  ore  within 
itself.  It  is  best  to  mix  the  weighed-out  portion  of  ore  with 
one-half  of  the  test  lead  to  be  used,  and  then  cover  over  with 
the  balance. 

The  scorification  may  be  divided  into  the  following  distinct 
steps: 

1.  Melting.     In  this  stage  the  lead  melts,  and  the  ore,  being 
of  a  lesser  gravity,  rises  to  the  surface  of  the  molten  lead  and 
floats  there. 

2.  Roasting.     The  ore  on  the  surface  of  the  lead  is  attacked 
by  the  oxygen  of  the  air  and  roasts  in  the  same  way  as  de- 
scribed under  "Roasting  of  Ores." 

3.  Scorification    Proper.     The    lead   commences    to    oxidize, 
forming  litharge.     A  small   percentage  (3)  volatilizes   and  the 
balance  forms   a  fusible   slag.     This  now  absorbs  the   oxides 
formed  by  the  roasting,  dissolving  them  and  forming  an  igneous 
solution.     The  silver  and  gold,  liberated,  are  absorbed  by  the 
remaining  metallic  lead.     The  slag,  as  it  forms,  drops  to  the 
side,  forming  a  slag  ring,  with  the  center  of  the  lead  bath  open 
to  the  atmosphere.     The  reason  for  this  is  that  the  meniscus  of 
molten  lead  is  convex,  thus  causing  the  collecting  of  the  slag 
on  the  rim  of  the  scorifier.     The  scorification  continues  until 
the  whole  of  the  lead  is  covered  over  with  slag.     It  is  then  con- 
sidered finished  and  the  assay  is  poured.     Should  the  assay  be 
left  in  the  muffle,  the  lead  will  still  continue  to  oxidize,  although 
none  is  exposed  to  the  air,  the  interchange  of  oxygen  taking 
place  by  means  of  the  litharge  and  other  oxides  present.     The 
size  of  the  lead  button  desired  from  this  assay  ranges  from  15 
to   20   grams.     If    the   scorification   is    continued   to   produce 
smaller  buttons,  losses   are  apt  to  occur  by  oxidation  of  the 
silver,  especially  if  this  is  present  in  considerable  amounts,  thus 
forming  rich  slags. 


124  A    MANUAL    OF    FIRE    ASSAYING 

The  temperature  of  scorification  ranges  from  1000°  C.  to  1100° 
C.,  although  with  pure  ores  higer  temperatures  may  be  employed. 

When  impure  ores  containing  much  base  metal  are  scorified, 
the  buttons  from  the  scorification  are  very  apt  to  be  contami- 
nated with  base  metal,  especially  copper,  and  will  then  have 
to  be  rescorified  with  more  test  lead,  in  order  to  get  a  pure 
button  for  cupellation. 

All  metals  are  to  some  extent  oxidized  simultaneously, 
but  a  mixture  of  metals  may  be  roughly  separated  by  successive 
oxidation,  each  metal  in  turn  partially  protecting  the  metal 
next  in  order,  while  the  latter  may  act  as  an  oxygen  carrier  to 
the  former.1  The  order  of  oxidation  is  as  follows: 

Fe  to  Fe2O3  Cu  to  Cu2O 

Zn  to  ZnO  Pt  to  — 

Pb  to  PbO  Ag  to  Ag2O 

Ni  to  Ni2O3  Au  to  AuO 

The  order  of  oxidation  of  the  following  elements  is  not  so 
certain: 

Sb  to  Sb2Os  Bi  to  Bi2O3 

As  to  As2O3  Te  to  TeO2 

C  to  CO,  S    toSOa 

The  order  given  in  the  table  shows  the  difficulty  encountered 
in  the  removal  of  copper  by  scorification,  as  lead  stands  ahead 
of  it  in  the  order  of  removal,  and  it  is  very  difficult  and  requires 
a  number  of  re-scorifications,  if  the  amount  of  copper  is  large, 
to  reduce  it  to  such  an  amount  as  to  prevent  loss  in  cupellation. 
Iron  and  zinc  are  very  readily  removed  by  scorification  (oxida- 
tion). Certain  elements,  like  Te  and  Se,  are  difficult  to  remove 
from  the  lead  button,  and  may  tend  to  concentrate  with  the  Au 
and  Ag  in  the  final  cupellation. 

The  slag  from  the  scorification  assay  should  be  homogeneous 
and  glassy.  If  it  has  an  earthy  appearance,  it  is  an  indication 
of  too  low  a  temperature  having  been  used,  and  the  button  is 
apt  to  be  brittle,  due  to  contained  PbO.  White  patches  of 
sulphate  of  lead  on  the  slag  after  pouring  also  indicate  rather 
too  low  a  temperatuie  of  scorification,  as  this  sulphate  forms  at 
a  low  temperature  under  slow  oxidation. 

The  scorification  method  is  a  reliable  one  on  most  materials, 
with  the  exceptions  enumerated  below.  As  the  usual  quantity 

1  T.  K.  Rose.  "Refining  Gold  Bullion,  etc.,  with  Oxygen  Gas,"  in  Trans.  I.  M.  M.,  April, 
1905. 


ASSAY    OF    ORES    CONTAINING    IMPURITIES  125 

taken  for  assay  is  0. 1  to  0.2  assay  ton,  it  is  evidently  not  a  suitable 
method  for  low-grade  ores,  especially  low-grade  gold  ores,  where 
at  least  0.5  to  1.0  assay  ton  must  be  taken  in  order  to  get  accurate 
results,  and  avoid  the  multiplication  of  the  error  of  weighing. 
It  is  practically  impossible  to  get  reliable  results  on  $5  to  $10 
gold  ores  by  ordinary  scorification.  If,  however,  10  assays  of 
0.1  assay  ton  are  made,  the  buttons  from  these  combined  and 
re-scorified  into  one  button,  which  is  then  cupelled,  the  results 
are  reliable,  but  not  so  good  as  from  the  crucible  assay  on  the 
same  total  amount,  on  account  of  the  multiplicity  of  weighing  and 
other  operations,  which  occasion  errors  and  losses.  The  method 
in  this  instance  would  also  be  more  costly  of  time  and  materials. 

For  ordinary  and  rich  silver  ores,  and  very  rich  gold  ores  or 
furnace  products,  such  as  bullions,  mattes,  etc.,  the  method  is  a 
desirable  one.  It  requires  no  preliminary  operations  and  thus 
saves  valuable  time.  The  slag  loss  is  frequently  somewhat  higher 
than  in  the  crucible  assay.  It  is,  as  ordinarily  performed  (in 
duplicate),  a  cheap  method  as  regards  fluxes,  etc.  It  does  not 
give  good  results  on  very  basic  ores,  i.e.,  those  containing  hema- 
tite, manganese  oxides,  etc.,  as  in  this  case,  unless  a  great  deal 
of  lead  is  used,  scoria  are  apt  to  form  in  the  slag,  which  may 
entangle  lead  and  undecomposed  ore.  Neither  does  it  give  good 
results  on  telluride  ores,  cyanide  precipitates,  or  ores  that  contain 
chloride  of  silver. 

When  basic  material  is  to  be  scorified,  small  additions  of  SiC>2, 
up  to  1  gram,  may  prove  advantageous.  In  general,  however,  the 
addition  of  fluxes,  except  test  lead,  is  not  to  be  recommended. 
Scorification  may  be  modified  by  the  addition  of  considerable 
amounts  of  borax  glass,  litharge,  silica,  when  it  approaches  the 
crucible  assay  in  character  with  none  of  its  advantages. 

THE  COMBINATION  METHOD.— The  trouble  arising  from  the 
presence  of  considerable  amounts  of  base  metals,  such  as  copper 
and  zinc,  has  been  fully  discussed  in  previous  pages,  as  well  as 
the  difficulty  of  their  removal  by  fusion  methods.  For  this 
reason  the  combination  wet-  and  dry-method  has  been  developed, 
to  remove  the  objectionable  impurities  by  solution.  The  method 
is  used  chiefly  on  copper-bearing  material,  such  as  heavy  copper 
ores,  copper  mattes,  blister  copper,  and  to  a  lesser  extent  on 
zinc  ores,  and  on  cyanide  precipitates  produced  by  zinc,  and 
has  been  advocated  for  telluride  ores. 

Van  Liew's  Method  for  Blister  Copper. — This  is  a  standard 


126  A   MANUAL    OP   FIRE    ASSAYING 

method  for  copper  material.  Weigh  out  duplicate  samples  of 
1  assay  ton  each  of  copper  borings,  add  350  c.c.  cold  water  and 
100  c.c.  HNO3  (sp.  gr.  1.42)',  and  set  in  a  cool  place  for  20  hours, 
stirring  from  time  to  time.  Then,  if  the  copper  is  not  dissolved, 
add  from  5  to  30  c.c.  more  of  concentrated  acid.  At  the  end  of 
26  to  28  hours  the  solution  of  the  copper  is  complete.  Do  not 
apply  heat  in  order  to  minimize  as  much  as  possible  the  solution 
of  small  quantities  of  gold,  by  whatever  action  this  may  take 
place.  The  oxides  of  nitrogen  in  the  solution  are  removed  by 
blowing  air  into  it  for  20  to  30  minutes. 

Salt  solution  (containing  0.54207  grams  of  NaCl  per  1000  c.c.) 
is  added  in  sufficient  quantity  to  precipitate  the  Ag  present  as 
chloride.  1  c.c.  of  this  solution  will  precipitate  1  mg.  of  Ag, 
and  an  excess  of  4  to  8  c.c.  above  that  required  for  the  Ag  should 
be  added.  If  the  amount  of  Ag  in  the  copper  is  small,  add  10  c.c. 
of  a  saturated  solution  of  lead  acetate  and  2  c.c.  of  concentrated 
H2SO4  in  order  to  form  PbSO4,  to  aid  in  settling  the  silver  chloride. 
Let  this  stand  for  about  12  hours  and  filter  the  precipitate  into 
the  proper  sized  filter,  and  wash  it  well  into  the  point  of  the  filter 
paper.  Dry  the  filter  carefully  in  the  air  bath,  and  when  dry, 
add  8  grams  of  test  lead  on  top  of  the  precipitate,  and  carefully 
transfer  to  a  scorifier  containing  2  grams  of  lead.  This  is  placed 
in  the  muffle,  heated  just  to  incipient  redness,  and  the  filter 
papers  burnt  off,  but  only  until  the  flame  disappears,  and  not 
into  ash.  This  takes  only  a  minute  or  so,  the  precaution  being 
taken  to  prevent  loss  of  silver  by  volatilization  as  AgCl,  the  lead 
and  carbon  present  reducing  the  AgCl  to  Ag.  Then  add  3  to 
4  grams  of  PbO,  and  the  same  amount  of  borax  glass,  raise  the 
heat  until  well  molten,  and  pour.  No  scorification  is  necessary, 
as  no  impurities  are  present.  The  lead  button  will  weigh  5  to 
8  grams  and  is  cupelled  with  feather  litharge.  The  results  should 
check  within  0.2  to  0.3  oz.  for  Ag  and  very  closely  for  gold.1 
Sulphuric  Acid  Method  for  Blister  Copper.2 — To  80  c.c.  of 
cone.  H2SO4  add  25  c.c.  of  a  solution  of  CuSO4  (160  grams  per 
1000  c.c.)  using  a  low  wide  No.  5  beaker.  Heat  to  such  a  tem- 
perature that  on  the  addition  of  the  copper  borings  action  com- 
mences immediately;  add  1  a.  t.  borings,  spreading  them  over  the 
bottom  of  the  beaker.  Heat  until  all  dissolving  action  has  ceased, 

1  R.  W.  Van  Liew,  in  Eng.  and  Min.  Jour.,  LXIX,  498  et  seq. 

2F.  F.  Hunt,   "Determination  of  Gold  in   Copper   Bullion,"    Eng.   and    Min.    Jour., 
LXXXVII,  465. 


ASSAY    OF    ORES    CONTAINING    IMPURITIES  127 

usually  from  1  to  1J  hours;  then  cool  and  add  400  c.c.  of  dis- 
tilled water,  stirring  to  prevent  caking  of  the  crystals.  Bring  to 
just  a  boil,  filter,  and  wash  the  beaker  thoroughly,  using  a  rubber- 
tipped  glass  rod  as  a  stirrer.  Place  the  filter-paper  with  the 
residue  in  a  2.5-in.  scorifier,  dry  and  burn  off  the  paper;  add  35 
grams  test  lead  and  1  gram  silica,  scorify  to  a  button  of  about  9 
grams,  cupel,  and  part  as  usual. 

Silver  may  be  determined  by  adding  salt  solution,  as  in  Van 
Liew's  method,  and  10  c.c.  of  a  10  per  cent,  solution  of  lead  acetate, 
stirring  well  and  letting  stand  over  night.  Then  filter  with  the 
usual  precaution,  and  add  the  paper  and  precipitate  to  the  same 
scorifier  containing  the  gold,  and  proceed  as  in  the  case  of  gold 
only. 

In  place  of  cupric  sulphate,  mercuric  nitrate  or  mercuric 
sulphate1  may  be  used,  the  equivalent  of  about  100  mgs.  of 
mercury  for  an  assay  ton  of  borings.  The  mercury  salt  is  best 
added  to  the  copper  borings,  stirring  a  little  and  then  adding  the 
80  c.c.  sulphuric  acid  and  boiling  on  a  hot  plate  for  three-quarters 
of  an  hour.  Then  proceed  as  already  described.  When  mercury 
salt  is  used  in  the  above  quantity  on  low-grade  bullions  containing 
from  10  to  50  oz.  Ag  per  ton  all  the  silver  is  thrown  down  with  the 
gold.  If  more  silver  is  present  salt  solution  should  be  added  in 
sufficient  quantity  to  precipitate  the  silver  and  any  mercury 
that  has  passed  into  solution. 

The  object  of  the  addition  of  cupric  sulphate  or  the  mercuric 
salt  is  to  prevent  the  formation  of  copper  sulphides,  which  will 
remain  in  the  residue  and  make  necessary  more  than  one  scorifi- 
cation  to  remove  the  copper  before  cupellation. 

The  sulphuric-acid  method  is  stated  to  give  results  equal  to 
the  "  all  fire  "  method  (p.  139)  on  gold. 

Combination  Assay  for  Matte. — Van  Liew's  method  of  treating 
in  the  cold  is  rarely  suitable  for  mattes,  as  heat  is  usually  essential 
in  order  to  insure  a  decomposition  of  the  matte  in  a  reasonable 
length  of  time.  Take2  duplicates  of  I  assay  ton  each  and  treat 
in  large  beakers,  provided  with  watch-glass  covers,  with  100  c.c. 
of  distilled  water  and  50  c.c.  HNO3  (sp.  gr.  1.42).  After  the 
violent  chemical  action  subsides,  add  50  c.c.  more  of  concentrated 
acid,  and  warm  the  beakers  on  a  hot  plate  until  everything 
soluble  is  dissolved:  usually  the  residue  is  white  or  grayish. 

1  F.  B.  Flinn,  Eng.  and  Min.  Jour.,  LXXXVII,  569;  also  M in.  and  Sci.  Press,  CI,  148. 

2  "Assay  of  Copper  and  Copper  Matte,"  in  Trans.  A.  I.  M.  E..  XXV.  258. 


128  A    MANUAL    OF    FIRE   ASSAYING 

Next  evaporate  a  considerable  part  of  the  acid  by  boiling,  ex- 
pelling all  of  the  nitrous  fumes,  dilute  to  500  c.c.,  add  3  c.c.  of 
concentrated  H2SO4,  10  e.g.  of  saturated  lead  acetate  solution, 
and  enough  salt  solution  of  the  strength  mentioned  for  blister 
copper  to  precipitate  the  silver;  then  stir  briskly  and  let  them 
stand  over  night.  Next  morning  warm  the  solutions  on  a  steam 
bath  and  filter  through  rather  thick  filter-paper. 

Filtrates  must  be  perfectly  clear  and  free  from  suspended 
PbSO4.  Wash  beakers  and  residue  thoroughly  with  hot  water, 
dry  the  filters  in  an  air  bath,  and  then  wrap  them  up  in  about  8 
grams  of  sheet  lead  and  scorify  with  40  grams  of  test  lead  and 
1  gram  of  borax  glass.  Cupel  the  buttons  with  feather  litharge. 
Re-assay  the  slag  from  the  scorification  and  the  cupel  and  add 
the  resultant  gold  and  silver  to  the  assay. 

When  heavy  copper  ores  are  to  be  assayed  by  this  method, 
which  are  apt  to  leave  large  amounts  of  silicious  residue,  the 
general  method  for  mattes  is  followed,  except  that  the  residues 
after  filtering  and  drying  are  treated  as  follows: 

Take  a  20-gram  crucible  and  place  in  it  i  assay  ton  of  PbO; 
then  put  the  filter-paper  containing  the  residue  on  top  of  this, 
place  the  crucible  in  the  mouth  of  the  muffle  at  a  low  heat,  burn 
off  the  filter-paper  until  the  flame  subsides,  remove  from  the 
muffle,  put  a  cover  on  the  crucible,  and  allow  to  cool.  When 
cold  add  0.5  assay  ton  PbO,  15  grams  of  Na2CO3,  2  grams  of 
argol,  mix  well  with  a  spatula,  and  put  on  a  cover  of  borax  glass. 
Then  proceed  as  in  the  ordinary  assay. 

General  Precautions  to  be  Observed  in  the  Combination  Assay. — 
The  combination  methods  on  copper  material  agree  well  with  the 
standard  scorification  methods  for  the  same  material  when  cor- 
rection of  cupel  loss  is  made  for  the  latter  method.  The  scori- 
fication methods  will  often  seem  to  give  higher  results,  but  this 
is  in  most  cases  due  to  the  fact  that 'the  silver  beads  frequently 
contain  from  2.5  to  4  per  cent,  copper.  The  combination  method 
gives  in  most  cases  (Van  Liew's  method  possibly  excepted) 
uniformly  lower  results  in  gold  (4  per  cent.)  than  the  standard 
corrected  scorification  method.  This  is  generally  ascribed  to  the 
formation  of  nitrous  acid  (HNO2)  during  solution,  which,  in 
connection  with  nitric  acid,  is  said  to  have  a  solvent  action  on 
gold;  but  such  authorities  as  W.  F.  Hillebrand1  dispute  this. 

1  W.  F.  Hillebrand  and  E.  T.  Allen,  "Comparison  of  a  Wet  and  Crucible-Fire  Methods 
for  Gold  Telluride  Ores,"  Bull.  253,  U.  S.  G.  Survey. 


ASSAY    OF    ORES    CONTAINING    IMPURITIES  129 

The  solution  may  be  due  to  the  formation  of  H2SO4  during  solu- 
tion, as  the  mixture  of  this  acid  and  HNO3  has  a  solvent  action, 
or  to  the  presence  of  impurities  like  chlorides  or  HC1,  etc.,  or 
possibly  to  the  presence  of  nitrates,  particularly  those  of  iron  or 
copper. 

It  has  been  demonstrated  that  gold  is  soluble  in  hydrochloric 
acid  solutions  of  iron  alum,  and  of  cupric  chloride,  but  not  in 
pure  HC1.1 

The  fact  that  the  combination  method  on  copper-bearing 
material  gives  low  results  on  gold  is,  however,  well  established. 

Owing  to  the  number  of  manipulations  in  the  combination 
assay,  it  is  often  apt  to  give  low  results  in  the  hands  of  inexpe- 
rienced chemists,  mainly  due  to  the  mechanical  losses  in  handling. 
The  directions  given  should  be  carefully  followed,  especially  those 
regarding  amount  of  solution,  strength  of  acid,  temperature, 
time,  etc.  Neatness  is  indispensable.  The  HN03  must  be  pure. 
The  directions  regarding  the  burning  off  of  the  filter-paper  must 
be  closely  followed.  The  amount  and  strength  of  the  salt  solu- 
tion must  be  carefully  adhered  to  and  it  must  be  added  at  the 
proper  time.  Some  assayers,  instead  of  adding  salt  solution  at 
the  same  time  as  H2SO4  and  Pb(C2H3O2)2,  filter  off  the  residue 
containing  the  gold  and  make  a  separate  precipitation  for  the 
silver,  believing  that  the  addition  of  a  salt  solution  may  cause  a 
slight  redissolving  of  the  gold.  At  this  point  of  the  assay  that 
is,  however,  hardly  probable.  A  large  amount  of  NaCl  is  to  be 
avoided,  as  AgCl  is  very  appreciably  soluble  in  brine.  C.  White- 
head  recommends  NaBr  or  KBr  instead  of  NaCl  for  this  reason. 

Combination  Method  for  Precipitates  from  the  Cyanide  Process.2 — 
Where  the  troublesome  base-metal  impurity  is  zinc  instead 
of  copper,  as  in  this  case,  sulphuric  acid  can  be  substituted  with 
advantage  for  HNO3.  The  method  is  as  follows: 

Of  the  precipitates  0.10  assay  ton  is  taken,  placed  in  a  beaker, 
and  20  c.c.  of  sulphuric  acid  (concentrated)  and  60  c.c.  of  water 
are  added.  This  is  heated  on  a  hot  plate  for  about  one  hour,  or 
until  zinc  and  zinc  oxide  are  in  complete  solution.  Add  salt 
solution  of  the  strength  already  mentioned  in  the  paragraph  on 
Van  Liew's  method  for  blister  copper,  in  slight  excess,  to  pre- 
cipitate the  silver  present,  remembering  that  1  c.c.  will  precipitate 

1  W.  J.  McCaughey,  Jour.  Am.  Chem.  Soc.,  XXXI,  1261. 

2  Fulton  and  Crawford,  "Notes  on  Assay  of  Zinc  Precipitates  Obtained  in  the  Cyanide 
Process,"  in  School  of  Mines  Quart.,  XXII,  153. 


130  A   MANUAL   OF   FIRE   ASSAYING 

1  mg.  of  silver.     Stir  briskly  with  glass  rod  to  agglomerate  the 
silver-chloride  formed. 

The  residues  are  then  filtered  through  the  proper  sized  filter, 
carefully  washed  with  hot  water  into  the  point  of  the  filter-paper, 
and  dried  in  the  air  bath  at  a  low  heat.  After  drying,  transfer 
to  a  20-gram  crucible  containing  1  assay  ton  of  litharge,  and  burn 
the  filter-paper  off  in  the  manner  already  described.  Then  add 
15  grams  of  soda  and  2  grams  of  argol,  mix  thoroughly,  and 
cover  with  a  heavy  cover  of  borax  glass.  Fuse  and  cupel  the 
resultant  lead  button.  Weigh  the  gold  and  silver  bead,  and 
from  a  preliminary  assay  determine  the  proper  amount  of  silver 
necessary  in  order  to  inquart  the  bead.  The  amount  of  silver 
should  be  just  about  2.5  times  the  amount  of  gold.  Roll  out  the 
bead,  after  flattening  with  a  hammer,  until,  after  repeated 
rollings,  the  fillet  will  have  about  the  thickness  of  a  visiting  card. 
It  is  best  to  anneal  the  bead  at  a  red  heat  between  the  various 
rollings,  in  order  to  prevent  cracking  on  the  edges.  Then  part 
in  a  parting  flask  in  hot  nitric  acid  having  a  specific  gravity  of 
1.26.  Boil  twice  for  at  least  20  minutes  each  time,  in  order  to 
insure  the  complete  remova  of  the  silver.  This  method  of 
parting  leaves  the  gold  in  one  coherent  mass,  termed  a  "  cornet," 
and  is  identical  with  the  method  practised  in  the  gold  bullion 
assay. 


CHAPTER  X 
SPECIAL  METHODS  OF  ASSAY 

TELLURIDE  ORES. — Gold  ores  containing  the  precious  metals  in 
the  form  of  tellurides  of  gold  and  silver,  mainly  calaverite  and 
sylvanite,  are  more  difficult  of  assay  than  ordinary  gold  ores, 
and  special  methods  are  essential  in  order  to  get  good  results. 
The  scorification  assay  is  not  reliable  for  telluride  ores,  giving 
almost  uniformly  low  results.  It  is  not  used  by  assayers  and 
chemists  of  the  great  telluride  ore  district  in  Colorado — Cripple 
Creek.  It  seems  that  in  scorification  the  main  cause  of  loss  is 
volatilization,  for  while  the  slag  loss  is  higher  than  for  ordinary 
ores,  slag  and  cupel  corrections  still  leave  the  results  from 
this  assay  far  below  those  of  the  crucible  assay  when  properly 
performed. 

Of  recent  years  selenium  gold  ores  have  been  found1  and  in 
general  the  precautions  necessary  for  the  assay  of  telluride  ores 
apply  also  to  selenium  gold  and  silver  ores. 

Tellurium  has  a  great  affinity  for  gold  and  silver  that  for 
silver  being  greater  than  that  for  gold;  and  if  a  high-grade 
telluride  ore  be  assayed,  even  by  special  method,  the  beads  from 
the  cupellation  will  frequently  still  contain  tellurium.2  In  the 
crucible  assay  the  losses,  which  are  somewhat  greater  than  in 
ordinary  ores,  occur  in  the  slag,  and  from  the  presence  of  the 
Te  in  the  lead  button,  causing  absorption  of  precious  metals 
by  the  cupel.  The  aim  in  the  crucible  assay  is  to  remove  the 
tellurium  from  the  gold  and  silver  and  slag  it.  This  is  best 
accomplished  by  the  presence  of  considerable  litharge  as  an  oxi- 
dizing agent,  and  otherwise  properly  balancing  the  flux.  The 
flux  recommended  quite  generally  by  Cripple  Creek  assayers  is 
made  up  as  follows: 

Potassium  carbonate 7      parts        Flour 1.0  parts 

Sodium  carbonate 6      parts         Litharge 30 . 0  parts 

Borax  glass 5.5  parts 

1  "Selenium  Gold  Ore,"  Eng.  and  Min.  Jour.,  XC,  418;  Min.  and  Sci.  Press,  C,  224. 

2  E.  C.  Woodward,  "Cupel  Losses  in  Telluride  Ores,"  in  West.  Chem.  and  Met.,  I,  120. 

131 


132  A    MANUAL    OF    FIRE    ASSAYING 

This  is  for  the  ordinary  silicious  Cripple  Creek  ores.  About 
75  grams  of  this  flux  is  used  with  0.5  assay  ton  of  ore.  This  gives 
the  following  charge: 

Ore 0.5  assay  ton  Borax  glass 8.5  grams 

PbO 45. 5  grams  Na2CO3 9.0  grams 

Flour 1.5  grams  K2CO3 10 . 5  grams 

The  heat  recommended  is  such  that  a  temperature  of  1063°  C., 
the  melting-point  of  gold,  is  reached  at  the  mouth  of  the  muffle. 
Some  assayers  recommend  a  soniewhat  greater  temperature  to 
insure  the  decomposition  of  the  tellurides.  The  time  of  fusion 
should  be  about  45  to  50  minutes. 

In  most  telluride  ores  the  silver  contents  are  not  great  enough 
to  permit  of  the  parting  of  the  bead  obtained  from  cupellation. 
It  is  therefore  necessary  to  add  silver  at  some  stage  before  part- 
ing and  in  this  instance  it  is  best  done  during  the  crucible  assay, 
since  by  doing  this  there  is  apt  to  be  less  absorption  of  gold  dur- 
ing cupellation  on  account  of  the  presence  of  silver  in  the  lead 
button. 

It  is  essential  to  recognize  that  the  flux  recommended  above 
for  tellurides  does  not  make  what  can  be  strictly  termed  an 
"excess-litharge  charge." 

Hillebrand  and  Allen1  recommend  the  following  charge  for 
Cripple  Creek  ores: 

Ore 1  assay  ton  Borax  glass 10  grams 

NaHCO3 1  assay  ton  Reducing  agent  (if  necessary) 

PbO 6  assay  tons  Salt  cover 

This  approaches  more  nearly  the  excess-litharge  charge. 

The  salt  as  a  cover  may  with  advantage  be  replaced  by  litharge. 
The  fusion  should  be  conducted  slowly  and  at  a  temperature  not 
exceeding  950°  to  1000°  C. 

It  is  essential  in  telluride  ores  to  have  the  sample  crushed  to 
120-  or,  better,  to  150-mesh.  The  reason  for  this  is  that,  owing 
to  the  irregular  distribution  of  values  in  these  ores,  fine  crushing 
is  required  to  get  a  true  sample,  and  also  because  the  low  melting- 
point  of  the  charge  usually  employed  makes  this  essential. 

The  precise  behavior  of  tellurium  in  the  crucible  assay,  and  dur- 
ing scorification  or  cupellation,  has  not  as  yet  been  investigated 

1  "A  Comparison  of  a  Wet  and  Crucible- Fire  Methods  for  the  Assay  of  Gold  Telluride 
Ores,"  Bull.  No.  253,  U.  S.  G.  Survey. 


SPECIAL   METHODS    OF    ASSAY 


133 


with  scientific  thoroughness  though  some  preliminary  work  has 
been  done.1  The  following  facts  are  reasonably  well  established: 
1.  The  great  affinity  of  tellurium  for  gold  and  silver,  resulting 
in  tellurium  passing  to  the  lead  button  with  the  precious  metals, 
unless  a  charge  be  used  that  is  essentially  oxidizing  in  its  char- 
acter, and  effecting  the  slagging  of  the  tellurium.  2.  During 
the  oxidation  of  the  lead  button  by  cupellation  or  scorification 
the  tellurium  tends  to  concentrate  in  the  remaining  lead-gold- 
silver  alloy  although  in  different  degree  in  the  two  operations,  the 
concentration  being  more  pronounced  in  scorification.  3.  The 
effect  of  tellurium  on  the  lead-gold-silver  alloy  is  to  very  greatly 
decrease  its  surface  tension,  so  much  in  fact  that  if  proportion- 
ately sufficient  tellurium  be  present  the  surface  tension  is  changed 
sufficiently  to  cause  the  alloy  to  "wet"  the  cupel  and  be  absorbed 
as  alloy,  thus  causing  heavy  losses  of  precious  metals.  If  the 
proportion  of  lead  to  tellurium  to  gold  should  attain  the  concen- 
tration of  10:1:1,  complete  absorption  may  take  place  leaving 
no  gold  bead.  Such  a  result  will,  however,  occur  only  in  excep- 
tional cases,  as  in  the  direct  cupellation  of  telluride  mineral,  etc. 
The  accompanying  data  due  to  S.  W.  Smith,  shows  the  relative 
elimination  of  tellurium  during  cupellation  and  scorification. 
Lead  buttons  of  20  grams  containing  0.05  gram  each  of  gold  and 
tellurium  were  submitted  to  cupellation  and  scorification,  and 
the  process  interrupted  at  intervals  for  the  determination  of 
tellurium. 


TABLE  XXIII.— SHOWING  ELIMINATION  OF  TELLURIUM  DUR- 
ING CUPELLATION  AND  SCORIFICATION 


Cupellation 


Scorification 


Cupelled          Per  cent,  of        Per  cent,  of    I        Scorified       |    Per  cent,  of 
down  to  wt.      original  lead       tellurium  in    ( I    down  to  wt.       original  lead   J  tellurium  in 


laining      ,     the  button 


remaining 


20.00 

100.00 

2.48 

20.00 

100.00 

2.48 

11.86 

59.00 

3.25 

6.145 

30.7 

5.94 

4.25 

21.00 

2.49 

2.085 

10.4 

12.90 

1.635 

8.15 

1.80 

1  S.  W.  Smith,  Trans.  I.  M.  M..  Buls.  44,  45  and  47  (1908).     Holloway  and  Pearse,  Trans. 
I.  M.  M.,  Buls.  39.  40,  and  45  (1907  and  1908). 


134  A   MANUAL    OF    FIRE    ASSAYING 

It  will  be  noted  that  during  cupellation  the  tellurium  at  first 
concentrates  in  the  lead  and  then  begins  to  be  eliminated,  the 
percentage  decreasing.  In  the  case  of  scorification  there  is  a  very 
decided  concentration  of  tellurium  in  the  lead  toward  the  end 
of  the  operation.  The  difference  is  probably  due  in  large  part 
to  the  fact  that  in  cupellation  the  tellurium  is  in  part  absorbed 
by  the  cupel  as  a  lead  tellurium  alloy,  which  action  cannot  take 
place  in  scorification.  The  removal  of  tellurium  by  oxidation 
from  lead,  thus  passing  into  cupel  or  slag,  is  evidently  a  difficult 
process.  In  an  alloy  of  lead,  gold  and  silver  and  tellurium,  the 
tellurium  can  form  compounds  with  both  the  precious  metals 
and  lead.  It  will  be  distributed  between  the  two  according  to 
the  relative  masses  present  and  the  relative  chemical  affinities. 
If  then  in  the  alloy  relatively  much  lead  be  present  (100  to  200 
parts  Pb  per  part  Au  and  Te)  by  far  the  larger  part  of  the  tellurium 
will  be  eliminated  by  absorption  as  lead  telluride,  and  only  a 
little  will  stay  with  the  precious  metals.  This  small  amount  will 
not  be  sufficient  to  materially  lessen  the  surface  tension  of  the 
bead  at  the  end  of  the  cupellation  and  hence  absorption  will  be 
small.  If,  however,  the  amount  of  lead  be  small  so  that  the 
relative  amount  of  tellurium  and  gold  be  increased  the  absorption 
of  the  latter  may  be  very  heavy.1  It  follows  therefore  that  in 
lead  buttons  obtained  in  the  crucible  assay,  which  may  contain 
tellurium,  it  is  better  to  cupel  directly  in  order  to  avoid  heavy 
absorption  in  the  cupel.  Scorification  might  be  resorted  to  in 
this  case  for  two  reasons:  1.  To  reduce  a  large  lead  button;  2. 
in  the  mistaken  idea  of  eliminating  tellurium. 

From  what  has  gone  before  it  is  evident  that  a  large  button 
is  not  disadvantageous  as  it  really  tends  to  decrease  the  absorp- 
tion of  precious  metal  when  tellurium  is  present. 

The  preliminary  scorification  of  lead  buttons  from  crucible 
assays  of  telluride  ores  has  been  shown  to  give  low  results.2  The 
cause  for  the  almost  universally  low  results  on  telluride  ores 
by  the  scorification  method  is  also  to  be  attributed  in  part  to  the 
above  reasons. 

4.  Silver  seems  to  exert  a  protective  action  on  the  gold  and 
lessen  the  absorption  of  the  latter,  due  probably  to  the  greater 
affinity  of  silver  for  tellurium,  thus  forming  silver  telluride  to  the 
exclusion  of  the  formation  of  much  gold  telluride,  consequently 

i  T.  K.  Rose,  Trans.  I.  M.  M.,  Bui.  40  (1908). 

3  C.  H.  Fulton.  School  of  Mines  Quart.,  XIX,  419,  and  S.  W.  Smith,  ibid. 


SPECIAL    METHODS    OF   ASSAY  135 

lessening  the  absorption  of  gold.  It  is  therefore  desirable  to 
perform  the  assay  in  the  presence  of  considerable  silver,  which 
will  have  to  be  added  anyway  at  a  later  stage  to  give  a  bead  that 
will  part. 

In  the  assay  of  telluride  ores  the  general  object  therefore  will 
be  to  remove  as  much  tellurium  from  the  gold  and  silver  before 
cupellation  as  is  possible.  This  is  best  done  by  the  performance 
of  a  crucible  assay  with  an  oxidizing  charge.  The  oxidizing 
properties  of  the  charge  are  obtained  by  the  use  of  an  excess  of 
litharge.  If  we  consider  the  ordinary  telluride  ore  as  composed 
of  a  silicious  or  shaly  gangue  containing  the  precious  metals  as 
tellurides  and  containing  also  certain  amounts  of  sulphides,  then 
when  this  is  subjected  to  fusion  with  litharge  (a  large  excess) 
the  telluride  minerals  and  sulphides  are  oxidized,  the  tellurium 
probably  forming  tellurate  of  lead  or,  in  the  presence  of  soda, 
tellurate  of  soda.  (See  behavior  of  sulphur,  Chapter  V.)  If, 
however,  an  insufficient  amount  of  PbO  is  present  so  that  it 
forms  lead  silicates  only  with  the  silica  of  the  ore,  the  oxidizing 
effect  will  be  much  diminished,  since  lead  silicates  form  at  a  low 
temperature  and  do  not  readily  give  up  oxygen.  It  is  therefore 
desirable  to  form  a  slag  which  has  the  characteristics  of  an 
excess  litharge  charge,  viz.,  is  not  glassy,  but  of  an  earthy  dull 
appearance.  Considerable  soda  should  be  present  to  aid  the 
oxidation  of  the  impurities.  The  borax  glass  should  not  exceed 
5  to  10  grams,  and  the  lead  button  made  should  be  large,  25  to  30 
grams.  The  fusion  should  be  made  slowly,  particularly  at  first, 
and  the  temperature  not  exceed  about  1000°  C.,  since  there  is  a 
possibility  of  dissociating  the  tellurium  compounds  in  the  slag 
and  sending  the  tellurium  into  the  lead  button. 

It  is  probably  impossible  to  remove  all  the  tellurium  from  gold 
and  silver  by  such  an  oxidizing  fusion  for  the  reason  that  the  re- 
duction of  lead  from  some  of  the  litharge  at  a  certain  stage  of  the 
assay  for  the  collection  of  the  gold  and  silver,  also  again  reduces 
some  of  the  tellurium  which  has  been  oxidized.  It  is  desirable  to 
obtain  the  full  oxidizing  effect  of  the  litharge  before  the  reduction 
of  lead  takes  place  and  for  this  reason  charcoal  is  to  be  recom- 
mended as  the  reducing  agent,  when  this  needs  to  be  employed, 
instead  of  argol  or  flour,  since  the  CO  evolved  by  the  two  latter 
begins  to  reduce  Pb  from  PbO  at  about  300°  C.  less  than  solid 
carbon,  which  acts  at  about  550°  C.  (page  64). 


136 


A    MANUAL    OF    FIRE    ASSAYING 


For  ordinary  silicious  telluride  ores  of   only  slight  reducing 
power  the  following  charge   is  recommended: 


Ore, 
PbO, 

Na2CO3, 

Borax  glass, 

Charcoal, 

Silver  foil, 

PbO  cover, 


0.5 

100.0 

30.0 

6.0 

1.1 


a.  t. 

gr. 

gr- 

gr- 

gr. 


10  to  20  mgs. 
10        gr. 


The  fusion  should  be  conducted  slowly  at  first,  the  final  tem- 
perature not  much  exceeding  1000°  C. 

If  the  button  from  the  fusion  is  thought  to  contain  tellurium, 
as  is  probably  the  case  in  the  assay  of  a  high-grade  ore,  it  will  be 
desirable  to  place  it  in  a  20-gram  crucible,  cover  with  30  grams 
PbO,  mixed  with  2  grams  borax  glass  and  bring  to  fusion,  then 
pour  and  proceed  as  usual.  This  treatment  will  eliminate  con- 
siderable tellurium  from  the  lead.  (S.  W.  Smith.) 

It  is  stated1  that  in  the  oxidizing  roasting  of  Cripple  Creek 
telluride  ores,  in  their  preparation  for  chlorination  or  cyanida- 
tion,  the  greater  part  of  the  tellurium  in  the  raw  ore  is  found  in 
the  roasted  ore  as  a  tellurite  of  iron.  Some  assayers  add  an  iron 
nail  to  the  assay,  not  so  much  to  desulphurize  as  to  provide  an 
excess  of  iron  for  the  purpose  of  combining  the  tellurium  with  it, 
as  in  the  case  of  sulphur. 

For  the  quantity  of  tellurium  present,  its  influence  on  the 
assay  is  certainly  profound.  The  following  table  gives  an  idea 
of  the  quantity  present: 

TABLE  XXIV.— QUANTITY  OF  TELLURIUM  IN  ORES 


Element 

Cripple  Creek  Ore 

Cripple  Creek  Ore 

Black  Hills 
Cambrian 

Black  Hills 
Cambrian 

Tellurium  
Gold  
Silver  

0.0742  per  cent.. 
0.0506  per  cent.. 
0.0075  per  cent.. 

0.092  percent.. 
0.060  percent.. 
0.0103  per  cent. 

0.0033  per  cent.. 
0.0026  per  cent.. 

0.010  percent. 
0.003  percent. 

As  already  stated,  tellurium  is  with  difficulty  separated  from 
gold  and  silver,  and  in  spite  of  an  oxidizing  charge  is  frequently 
carried  down  in  the  lead  button.  The  loss  then  takes  place  in 
the  cupel,  tellurium  causing  a  heavy  absorption.  Some  loss, 
however,  takes  place  by  volatilization.  There  is  also  a  somewhat 

1  Trans.  I.  M.  M.,  Ill,  49,  50. 


SPECIAL    METHODS    OF    ASSAY  137 

higher  slag  loss  in  the  telluride  assay  than  in  the  assay  of  ordinary 
ores.1  Hillebrand  and  Allen,  already  quoted,  assayed  telluride 
ores  by  the  combination  wet-and-dry  assay,  getting  the  gold  and 
silver  free  from  tellurium,  but  found  that  the  crucible  assay  as 
ordinarily  performed  for  telluride  ores  gave  just  as  satisfactory, 
if  not  better,  results. 

A  STUDY  OF  THE  ASSAY  OF  BLACK  HILLS  CAMBRIAN  ORES. 

— These  ores  are  probably  complex  tellurides.     The  ores  were 

oxidized  and  of  the  following  average  composition: 

Si02  =  71.5  per  cent.;  Fe2O3  =  16.3  per  cent.;  A12O3 =4.8  per  cent.; 

CaO  =  l.o  per  cent.;  Gold  =  0.79  oz.;  Ag  =  0.10  oz. 
Samples  of  this  type  of  ore,  representing  controls  on  car-load 
lots,  were  assayed  by  assayers  A  and  B  in  the  same  laboratory, 
with  the  same  kind  of  cupels,  and  great  regard  to  temperature  of 
cupellation.  Assayer  A  made  fusions  on  one  assay  ton  lots,  in 
triplicate,  with  the  following  stock  flux: 

Na2eO3 3. 25  parts         Borax  glass ...    5. 00  parts 

K2CO3 2. 25  parts         Argol 1.00  parts 

PbO 18. 00  parts 

29 . 50  parts 

The  amount  of  flux  used  was  4  assay  tons  per  assay  ton  of 
ore,  with  quite  a  heavy  borax  glass  cover.  Fusions  made  at 
1100°  C.,  approximately. 

The  stock  flux  is  equivalent  to  the  following  charge: 

Ore 1      assay  ton         PbO 73 . 2  grams 

Na2CO3 13. 2  grams  Borax  glass 20. 3  grams 

K2COj 9.1  grams  Argol2 4.0  grams 

On  account  of  the  negligible  quantity  of  Ag  present,  every 
assay  was  salted  with  Ag.  The  beads  were  parted  in  acid  1  to  9, 
and  were  in  each  case  required  to  check  against  each  other  in 
weight.  The  beads  were  then  weighed  together  and  the  resultant 
weight  divided  by  3  to  obtain  the  amount  of  gold. 

Assayer  B  made  assays  on  the  same  pulp  samples  with  the 
following  stock  flux: 

Na2CO3 3.25  parts         Borax  glass 2.00  parts 

K2CO3 2. 25  parts         Argol 0.75  to  1 .00  part 

PbO 22. 00  parts 

30. 25  parts 

1C.  H.  Fulton,  School  of  Mines  Quart.,  XIX.  F.  C.  Smith  Trans.  I.  M.  M..IX.  344- 
Min.  Rep.,  LI,  163.  Hillebrand  and  Allen,  Bull.  No.  253,  U.  S.  G.  Survey,  12,  14. 

2  This  amount  of  argol  required  because  ores  are  oxidizing.  The  button  produced 
usually  22  to  25  grams. 


138  A    MANUAL    OF    FIRE    ASSAYING 

Three  assay  tons  of  flux  were  used  to  each  0.5  assay  ton  of 
ore,  with  a  soda  cover  one-quarter  inch  thick.  Assays  were  made 
in  quadruple,  all  fusions  being  salted  with  Ag,  parted  in  1  to  4 
acid,  and  the  beads  required  to  check  against  each  other  in  weight 
and  then  weighed  together,  and  the  sum  divided  by  2  to  get  the 
value  per  ton. 

The  stock  flux  is  equivalent  to  the  following  charge: 
,  Ore 0.5  assay  ton        PbO 67      grams 

Na2CO3 9      grams  Borax  glass 6      grama 

K2CO3 6      grams  Argol  2.5  grams 

The  results  of  these  series  of  assays  were  as  follows: 

LOT  No.  ASSAYER  A  ASSAYER  B 

Oz.  Au  per  ton  Oz.  Au  per  ton 

88870 0.74 0.78 

88832 0.80 0.83 

88874 0.71 0.80 

88823 0.82 0.98 

88721 0.80 0.88 

88851 0.85 0.89 

88818 0.85 0.91 

88940 0.79 0.84 

3669 0.82 0.91 

88853 0.85 0.91 

88890 0.81 0.83 

71957 0.79 0.82 

88826 0.77 0.82 

3843 0.77 0.81 

88780 0.80 0.88 

22522 0.66 0.73 

98509 0.69 0.79 

22050 0.50 0.58 

Assayers  A  and  B  then  exchanged  fluxes,  and  as  they  checked 
each  other's  previous  results  closely,  it  became  evident  that  the 
flux  of  assayer  A  was  ill-balanced  and  would  not  give  good  results. 
Slag  and  cupel  corrections  were  made  by  Assayer  A  on  assays 
made  with  his  flux,  but  even  these  corrections  added  failed  to 
bring  his  results  up  to  those  of  assayer  B. 

The  question  arises  as  to  what  is  the  specific  trouble  with 
flux  A.  On  examination,  it  will  be  found  to  contain  an  excessive 
amount  of  borax  glass,  especially  when  the  cover  is  considered. 
It  is  very  probable  that  the  acidity  of  the  charge  (although  a  good 
fluid  slag  is  obtained)  is  so  great,  taking  into  account  both  the 
silica  of  the  ore  and  the  borax  glass,  that  the  ore  is  not  com- 


SPECIAL    METHODS    OF   ASSAY  139 

pletely  decomposed  by  the  basic  ingredients  of  the  charge;  i.e., 
the  soda  and  litharge  become  saturated  with  borax  and  then  do 
not  completely  decompose  the  silicious  ore.  The  fact  that  re- 
assays  of  the  slag  do  not  bring  the  results  up  to  the  figures  ob- 
tained by  assayer  B  does  not  necessarily  imply  that  the  slag  does 
not  contain  these  values,  as  the  charge  used  to  flux  the  slags  and 
cupels  again  contains  much  borax  glass,  so  that  practically  the 
same  conditions  obtained  as  before. 

THE  ASSAY  OF  COPPER-BEARING  MATERIAL. — Copper-bear- 
ing material  includes  ores  containing  copper  and  furnace  products, 
chiefly  mattes,  blister  copper,  etc.  Copper,  which  in  the  assay 
has  a  strong  tendency  to  go  into  the  lead  button,  causes,  when 
present  in  sufficient  quantity,  serious  losses  by  cupel  absorption. 
Therefore  all  methods  of  assays  for  this  class  of  material  endeavor 
to  eliminate  copper  from  the  lead  button  to  be  cupelled.  A 
standard  method  for  the  assay  of  material  high  in  copper,  espe- 
cially for  Ag,  is  the  combination  assay  for  blister  copper  and 
mattes,  described  in  Chapter  IX. 

Another  standard  method,  especially  for  gold,  and  one  that 
is  carried  out  frequently  as  a  check  to  the  above,  is  the  scorifica- 
tion  or  "all  fire"  method.  This  is  performed  as  follows: 

Ten  samples,  of  0.10  assay  ton  each,  are  taken  and  placed  in 
3-inch  scorifiers  with  50  grams  of  test  lead  (the  silver  content  of 
which  is  accurately  known) ;  25  grams  of  the  lead  are  mixed  with 
the  matte,  or  borings,  etc.,  and  the  other  25  grams  used  as  a 
cover.  On  top  of  the  charge  is  placed  1  gram  each  of  silica  and 
borax  glass.  The  scorification  is  carried  on  at  a  moderate  tem- 
perature until  the  assays  are  just  about  to  slag  over,  which  takes 
usually  about  25  minutes,  and  then  they  are  poured.  The  result- 
ant button  will  weigh  about  15  to  16  grams  and  be  quite  hard 
with  copper.  The  buttons,  cleaned  from  slag,  are  scorified,  test 
lead  being  added  to  make  the  total  lead  up  to  40  grams.  The 
second  scorification  will  take  about  30  minutes  and  the  resultant 
buttons  will  weigh  from  10  to  12  grams.  These  are  cupelled  in 
10  separate  cupels,  placed  so  as  to  be  subject  to  uniform  tem- 
perature, i.e.,  in  one  horizontal  row  across  the  muffle.  Cupella- 
tion  should  be  conducted  at  as  low  a  temperature  as  is  feasible. 

The  beads  are  weighed  separately  and  then  together.  They 
are  then  grouped  in  two  lots  of  5  each,  which  are  parted  in  acid, 
strength  1  to  9,  the  beads  being  kept  in  this  acid  at  nearly  boiling 
temperature  for  20  minutes  and  finished  for  5  minutes  with 


140  A   MANUAL    OF    FIRE   ASSAYING 

1.42  sp.  gr.  acid  (full  strength).  The  ten  cupels  are  taken  in  lots 
of  two  each  (only  the  litharge-stained  part  is  taken),  crushed  to 
pass  100-mesh  and  assayed  by  the  following  charge: 

100  grams  PbO  45  grams  borax  glass 

20  grams  Na2CO3  3  grams  argol 

Soda  cover 

The  lead  buttons  are  cupelled,  and  the  silver  and  gold  obtained 
added  to  the  first  weights.  The  scorification  slags  may  also  be 
reassayed  and  this  correction  added,  but  in  practice  the  cupel 
correction  is  the  only  one  usually  allowed.  Sometimes  no  correc- 
tion is  allowed.  It  is  to  be  noted  that,  even  with  a  rescorifica- 
tion  of  the  first  button  of  the  assay,  the  final  silver  beads,  from 
55  per  cent.  Cu  matte  containing  180  oz.  Ag  per  ton  and  2.31  oz. 
gold,  will  contain  from  2.5  to  4  per  cent.  _ copper,  which  must  be 
deducted  in  order  to  get  correct  silver  results.  (For  a  further 
discussion  of  scorification  slag  losses  and  cupel  absorption  in 
assaying  copper-bearing  material,  see  Chapter  XI.) 

The  scorification  method  is  generally  employed  for  the  deter- 
mination of  gold  in  mattes,  and  the  combination  method  for  the 
determination  of  silver.  Of  recent  years,  special  crucible  methods 
for  copper  mattes  and  copper-bearing  material  have  been  devel- 
oped with  considerable  success.1 

A  satisfactory  method  on  copper  mattes,  up  to  20  per  cent, 
copper  and  high  in  gold  and  silver,  was  practised  by  the  Standard 
Smelting  Company,  at  Rapid  City,  S.  Dak.  The  matte  sample 
is  put  through  a  120-mesh  screen,  and  for  controls  4  assays  of 
0.25  assay  ton  each  are  made,  with  the  following  stock  flux: 

Silica 11  parts         Sodium  carbonate 25  parts 

Litharge 70  parts         Niter 5  parts 

An  0.25  assay  ton  matte  is  run  with  a  3.5  assay  ton  flux  and 
a  thin  borax  glass  cover.  The  flux  figured  to  the  charge  is  as 
follows: 

0.25  assay  ton matte  24.0  grams Na2CO3 

10.5  grams SiO8  5.0  grams KNO3 

67.0  grams PbO 

The  heat  used  is  high  and  the  fusion  short,  giving  a  clean 
fluid  slag  and  a  bright  button  of  approximately  20  grams.  These 
buttons  are  cupelled  directly  for  gold  and  silver.  One  cupel  and 

»"An  All-fire  Method  for  the  Assay  of  Gold  and  Silver  in  Blister  Copper,"  in  Trans. 
A.  I.  M.  E.,  XXXIII  670.  Perkins.  "The  Litharge  Process  for  the  Assay  of  Copper- 
bearing  Ores,"  ibid.,  XXXI,  913. 


SPECIAL    METHODS    OF    ASSAY 


141 


one  slag  are  then  re-run  in  the  same  crucible  that  the  original 
fusion  was  made  in,  and  the  result  of  the  four  corrections  added 
to  the  sum  of  the  original  buttons.  No  scorification  is  made 
before  cupellation.  The  average  correction,  on  the  usual  grade 
of  matte  (5  oz.  Au,  40  oz.  Ag),  is  2.5  per  cent,  gold  and  5.5  per 
cent,  silver.  Below  is  a  comparison  of  this  method  with  the 
standard  scorification  assay,  including  cupel  and  slag  correc- 
tion. The  copper  content  of  this  matte  was  19.98  per  cent. 

TABLE  XXV.— COMPARISON  OF  METHODS  IN  ASSAY          * 


Crucible  method 

Scorification  method 

Gold  (oz.) 

Silver  (oz.) 

Gold  (oz.) 

Silver  (oz.) 

4.10 
0.10 

36.24 
1.00 

3.90 
0.25 

35.07 
1.95 

Correction 

4.20 

37.24 

4.15 

37.02 

The  returns  on  this  pulp  by  the  refiner  were:  gold,  4.19  oz.; 
silver,  36.71  oz. 
Matte  No.  1545;  copper  content,  17.6  per  cent. 


TABLE  XXVI.— COMPARISON  OF  METHODS  IN  ASSAY 


Crucible  method 

Scorification  method 

Gold  (oz.) 

Silver  (oz.) 

Gold  (oz.) 

Silver  (oz.) 

Original  assay  
Correction 

3.42 
0.10 

31.94 
1.85 

3.40 
0.11 

31.86 
1.93 

3.52 

33.79 

3.51 

33.79 

The  following  table  shows  results  by  this  method  with  correc- 
tion and  refiners'  results  (by  same  method  without  correction) : 


142 


A    MANUAL    OF    FIRE    ASSAYING 


TABLE  XXVII.    CORRECTED  AND  UNCORRECTED  ASSAYS 
ON  COPPER  MATTE 


Lot  No. 

Crucible  method: 
Standard  Smelting 
Company 

Crucible  method: 
Refiner 

Gold 

Silver 

Gold 

Silver 

Copper 

oz.  per  ton 

oz.  per  ton 

oz.  per  ton 

oz.  per  ton 

% 

17.73 

75.4 

17.67 

74.13 

7.2 

1404  

17.75 

73.5 

17.625 

72.42 

9.5 

1412  

11.35 

45.7 

11.145 

42.95 

10.3 

1435  

10.02 

39.9 

10.065 

39.08 

12.9 

1450  

9.10 

44.6 

9.02 

43.01 

13.2 

1457  

6.89 

48.75 

6.815 

46.28 

15.02 

1458 

9.95 

58.37 

9.935 

52.31 

19.9 

1470 

8.235 

50.86 

8.24 

52.31 

20.5 

1471  

4.845 

40.52 

4.87 

39.48 

18.4 

1484  

7.34 

45.04 

7.265 

44.68 

18.27 

1489  

6.45 

47.98 

6.83 

46.67 

19.4 

1500  

5.78 

45.34 

5.84 

45.06 

20.5 

1503 

4.61 

27.11 

4.58 

26.51 

18.8 

1513 

3.815 

32.75 

3.80 

31.55 

11.8 

1525  

3.12 

39.62 

3.205 

33.84 

12.4 

1529  

3.10 

33.00 

3.36 

31.14 

18.7 

1533  

4.20 

37.24 

4.19 

36.71 

20.0 

.4.10  oz.  per  ton 

Silica  

.  .  .  .      3.3  per  cent. 

31.55  oz.  per  ton 

Lime  

.  .  .  .     0.5  per  cent. 

17.4  per  cent. 

Sulphur.  .  .  . 

.  .  .  .   29.1  per  cent. 

45  .  9  per  cent. 

Lead  

.  .  .  .  trace 

2  .  5  per  cent. 

A  typical  sample  of  matte  on  which  these  assays  were  made 
analyzes  as  follows: 


Gold 

Silver 

Copper... 
Iron .  . 


The  crucible  charge  employed  can  readily  be  modified  to 
apply  to  mattes  higher  in  copper  or  greater  in  reducing  power. 

Perkins'  excess-litharge  method  has  already  been  described. 
He  states  that  for  low-grade  copper-bearing  material  (2  to  4  per 
cent.),  5  assay  tons  of  PbO  to  0.5  assay  ton  of  ore  will  remove 
most  of  the  copper,  if  the  balance  of  the  fluxes  is  properly 
proportioned,  i.e.,  if  there  is  ample  free  PbO  to  dissolve  copper 
oxides.  For  high-grade  mattes,  etc. — 48  to  60  per  cent,  copper 


SPECIAL   METHODS    OF   ASSAY  143 

—  8  assay  tons  of  PbO  to  0.  1  assay  ton  of  matte  will  remove  most 
of  the  copper.  Perkins  also  developed  a  crucible  method  for 
metallic  copper,  as  follows: 

Weigh  out  0.25  assay  ton  of  copper  borings,  divide  it  into 
3  approximately  equal  parts,  and  place  in  20-gram  crucibles.  In 
this  way  weigh  out  4  sets,  getting  12  assays.  Into  each  crucible 
put  800  mgs.  of  powdered  sulphur,  mix  thoroughly  with  the 
copper,  and  then  on  top  of  this  put  the  following  flux,  being 
careful  not  to  mix  the  flux  with  the  copper: 


0.25  assay  ton         PbO  ............   8.0  assay  ton 

K2CO3  .............   0.25  assay  ton         SiO2  ............   0.5  assay  ton 

Salt  cover 

Place  the  crucibles  into  a  dark-red  muffle  and  gradually  raise 
the  temperature  for  45  minutes  to  a  yellow  heat.  The  tempera- 
ture regulation  is  important,  and  it  is  necessary  to  produce  a 
neutral  or  reducing  atmosphere  in  the  muffle  by  the  presence  of 
coal  or  coke.  The  buttons,  weighing  about  18  grams  each,  are 
put  together  in  lots  of  three,  representing  0.25  assay  ton,  and 
scorified  at  a  low  heat.  The  resultant  buttons  should  weigh 
5  to  6  grams.  Each  of  these  buttons  is  now  rescorified  with 
25  grams  of  lead  at  a  low  heat,  until  6-gram  buttons  are  obtained. 
These  are  cupelled  with  feathers.  This  method  is  stated  to  give 
results  on  gold  equal  to  the  all-scorification  method,  and  on  silver 
equal  to  the  combination  method. 

THE  ASSAY  OF   ZINCIFEROUS  ORES  AND  METALLURGIC 

PRODUCTS  CONTAINING  ZINC.—  Zinc  most  frequently  occurs 
in  ores  as  the  sulphide,  sphalerite,  and,  in  certain  metallurgical 
products,  as  the  metal  (zinc  cyanide  precipitates)  .  Zinc  boils  at 
940°  C.,  and  rapidly  volatilizes.  Zinc  oxide  volatilizes  slowly  at 
1180°,  and  rapidly  at  1400°.  Zinc  silicates  alone  are  difficultly 
fusible,  but  are  readily  so  when  mixed  with  borax  or  boricacid 
or  ferrous  silicate.1  The  presence  of  zinc  in  material  to  be  assayed 
calls  for  certain  precautions,  and  in  general  the  assay  is  difficult. 
Metallic  zinc  has  a  great  affinity  for  gold  and  silver,  greater  than 
lead,  as  is  shown  by  the  Parkes  process  for  the  desilverization  of 
lead  bullion.  Under  oxidizing  influences2  the  formation  of  zinc 
oxide  and  its  volatilization  causes  losses  of  gold  and  silver.  That 
this  loss  is  mechanical  does  not  make  it  less  serious.  The  boil- 

1  Rose.  "Refilling  Gold  Bullion,  etc.,  in  Oxygen  Gas,"  in  Trans.  I.  M.  M.,  April,  1905. 

2  Williams,  in  Jour.  Chem.  Met.  and  Min.  Soc.  of  South  Africa,  III,  132. 


144  A   MANUAL    OF    FIRE    ASSAYING 

ing-point  of  zinc  occurs  at  a  temperature  somewhat  below  the 
normal  for  ordinary  scorification,  and  it  is  this  fact,  coupled  with 
the  fact  that  the  zinc  oxide  formed  is  with  difficulty  soluble  in 
litharge,  that  make  accurate  assay-results  hard  to  obtain,  especially 
in  scorification.  Zinc  containing  gold  and  silver  may  be  distilled 
off  and  volatilized  with  very  little  loss  of  gold  and  silver,  if  the 
conditions  are  reducing.1 

Scorification  is  frequently  employed  for  zinciferous  ores, 
although  it  is  not  generally  satisfactory.  When  used,  it  is  best 
carried  out  in  a  way  similar  to  that  adopted  for  copper-bearing 
material,  using  from  0.05  to  0.10  assay  ton  of  ore  with  from 
50  to  80  grams  of  test  lead,  2  grams  of  borax  glass,  and  1  gram 
of  silica,  the  last  being  essential  to  flux  the  zinc  oxide  formed. 
Otherwise  insoluble  scoria  and  crusts  form  on  the  scorifier.  Slag 
and  cupel  corrections  are  generally  necessary  and  from  5  to  10 
assays  are  made,  the  results  being  averaged.  As  zinc  is  readily 
oxidized,  lead  buttons  contaminated  with  zinc  are  not  to  be  feared 
and  rescorification  is  rarely  necessary.  Among  the  most  im- 
portant zinciferous  material  presented  for  assay  are  the  zinc-gold 
precipitates  from  the  cyanide  process.  Scorification  is  not  de- 
sirable for  these.2  They  are  best  assayed  by  the  crucible  method 
or  by  one  of  the  combination  methods  already  described. 

Crucible  Method. — The  crucible  method  best  suited  for  unox- 
idized  zinc  ores  is  the  niter  method,  with  sufficient  silica  present 
to  form  at  least  the  monosilicate  with  zinc.  Borax  glass  and 
much  litharge  is  also  desirable.  On  a  practically  pure  sphalerite 
the  following  charge  will  give  good  fusions  at  temperatures  of 
about  1100°  C.: 

Ore 0.5  assay  ton         SiO2 8  grams 

Na2CO3 15      grams  KNO, 22  grams 

PbO 150      grams  Heavy  borax  glass  cover.3 

This  charge  can  be  modified,  as  regards  niter  and  silica,  to 

suit  any  sphalerite  ore. 

A  good  crucible  charge  for  cyanide  precipitates,  containing  up 

to  50  per  cent,  zinc,  is: 

Precipitates 0.1  assay  ton  SiO2.' 5  grams 

Na2CO8 5      grams  Na2B4OT 2  grams 

PbO 70      grams  Flour 1  gram 

Light  borax  glass  cover 

1  Rose,  ibid.,  and  references. 

2  "Notes  on  the  Assay  of  Zinc  Precipitates,  etc.,"  in  School  of  Mines  Quart.,  XXII,  153. 

3  A  similar  charge  is  recommended  by  Lay,  for    complex  zinc-lead  concentrate;  see 
Min.  Ind.,  XIII.  287. 


SPECIAL   METHODS    OF   ASSAY  145 

The  following  method1  is  used  on  cyanide  precipitates  containing 
12,000  to  22,000  oz.  Ag,  and  300  oz.  Au  per  ton  at  the  mill  of 
the  N.  Y.  and  Honduras  Rosaria  Min.  Co.  in  Honduras,  C.  A. 

In  a  20  gram  crucible  mix  27  grams  test  lead,  2.5  grams  borax 
glass,  0.5  gram  silica  with  0.1  a.  t.  of  the  precipitates,  tapping  the 
crucible  to  make  certain  that  no  material  adheres  to  the  sides. 
In  another  crucible  mix  33  grams  of  PbO,  25  grams  of  Na2C03, 
4.5  grams  borax  glass,  1.5  grams  of  silica,  and  0.15  grams  charcoal. 
After  mixing  transfer  this  second  charge  on  top  of  the  contents  of 
the  first  crucible,  and  make  the  fusion  as  usual.  The  button 
separates  very  cleanly  from  the  slag.  The  slag  and  cupel  are 
reassayed,  and  one  weighing  made  on  the  three  beads  recovered, 
from  the  cupellation.  The  slag  corrections  are  small,  amounting 
to  about  25  oz.  Ag  per  ton  of  precipitates  containing  14,000  oz. 
The  gold  loss  in  the  slag  is  very  small. 

The  assays  are  run  in  triplicate.  In  the  assay  of  such  high- 
grade  material  weighing  of  precipitates  must  be  done  on  anal- 
ytical balances. 

Assay  of  Plumbago  Crucibles  for  Gold  and  Silver. — Graphite 
or  plumbago  crucibles  are  extensively  used  in  the  smelting  of 
cyanide-zinc  precipitates,  and  the  old  discarded  ones  are  usually 
sold  in  lots  to  some  smelter;  they  often  contain  considerable  gold 
and  silver.  These  pots  present  difficulty  in  assaying,  chiefly  on 
account  of  the  graphite  and  zinc  contained.  From  a  given 
weight  of  sample,  the  metallics  and  scales  are  separated  by 
passing  the  material  through  a  150-mesh  screen,  and  a  regular 
scale  assay  is  made  as  outlined  at  the  end  of  this  chapter.  The 
pulp  is  assayed  as  follows:2 

From  0.05  to  0.10  assay  ton  is  taken  and  mixed  with  a  little 
more  than  one-half  its  weight  of  niter  and  30  grams  of  litharge, 
placed  in  a  2.5  in.  scorifier,  covered  with  30  grams  of  litharge  and 
afterward  with  a  thin  cover  of  borax  glass,  placed  in  a  muffle, 
and  fused  finally  at  a  yellow  heat.  The  buttons  are  cupelled, 
weighed,  and  parted  as  usual.  Crucible  assays  may  also  be 
made  on  this  material  by  the  niter  excess-litharge  fusion,  with 
a  charge  as  follows: 

0.1  assay  ton  graphite  5  grams  Na2CO3 

70      grams  PbO  5    o  11  grams  KNO3  (according  to 

5      grams  SiO2  carbon  contents  of  pulp) 

Borax  glass  cover 

1  Private  communication,  E.  Van  L.  Smith      Name  of  the  originator  of  the  method  not 
known  to  the  author. 

2  A  modification  of  T.  L.  Carter's  method;  see  Eng.  and  Min.  Jour.  LXVIII,  155. 

10 


146  A   MANUAL    OF    FIRE    ASSAYING 

In  both  methods  it  is  essential  that  the  amount  of  pulp, 
usually,  should  not  exceed  0.1  assay  ton,  the  carbon  giving 
difficulties  with  greater  amounts  than  this. 

The  Assay  of  Residues  from  Zinc  Distillation  (containing  con- 
siderable carbon)  for  Silver  and  Gold.1 — From  0.10  to  0.5  assay 
ton  of  the  powdered  residue  is  mixed  with  35  grams  of  niter  and 
10  grams  of  Na2O2  (sodium  peroxide),  and  dropped,  in  lots  of 
5  grams  each,  into  a  red-hot  crucible  which  can  be  readily  covered, 
and  the  oxidation  reactions  permitted  to  complete  themselves. 
The  flux  then  added  consists  of  70  grams  of  litharge,  10  grams  of 
borax  glass,  10  grams  silica,  2  grams  argol  and  a  light  borax  glass 
cover.  The  fusion  is  carried  out  at  a  yellow  heat  and  the  buttons 
cupelled  as  usual. 

THE  ASSAY  OF  ANTIMONIAL  AND  ARSENICAL  ORES  FOR 

GOLD  AND  SILVER. — Gold-  and  silver-bearing  antimonial  ores, 
such  as  stibnite,  jamesonite,  etc.,  are  usually  assayed  by  the 
niter  method,2  in  the  presence  of  considerable  soda  and  niter,  to 
induce  the  formation  of  the  antimoniate  of  soda.  A  preliminary 
assay  to  determine  the  amount  of  niter  is  essential.  The  follow- 
ing charge  is  recommended  for  nearly  pure  stibnite:3 

Ore 0.5  assay  ton         KNO3 18  grams 

PbO 120      grams  Borax  glass 6  grams 

Na2CO3 10      grams  SiO2 10  grams 

Salt  cover 

The  fusion  should  be  conducted  slowly  and  at  a  low  tempera- 
ture. The  button  will  usually  contain  very  little  antimony,  the 
cupel  not  showing  scoria  or  cracks.  If  it  does  contain  enough 
to  cause  losses  in  cupellation,  the  buttons  should  be  scorified. 
Smith4  gives  the  following  charge  for  ore  containing  ap- 
proximately 75  per  cent,  stibnite.  The  niter,  etc.,  can  be 
varied  for  the  ore  as  the  gangue  increases: 

Ore 1  assay  ton         Borax  glass 8  grams 

PbO 75  grams  KNO3 20  to  25  grams 

Na2CO3 25  grams  Salt  cover. 

Another  method,  practically  as  good  as  the  niter  method,  is 
the  roasting  with  charcoal  or  coke-dust.5  The  sample  of  ore, 

1  K.  Sander,  in  Eng.  and  Min.  Jour.,  LXXIII,  380. 

2  William  Kitto.  "The  Assay  of  Antimonial  Gold  Ores,"  in  Trans.  I.  M.  M.,  1906,  Nov.  8 
and  Dec.  13. 

8  Smith,  "The  Assay  of  Complex  Gold  Ores."  in  Trans.  I.  M.  M.,  IX,  332. 

4  Smith,  ibid. 

6  Sulman,  Trans.  I.  M.  M.,  IX,  340. 


SPECIAL   METHODS    OF   ASSAY  147 

usually  i  assay  ton,  is  mixed  with  approximately  its  own  volume 
of  coke-dust  or  coal-dust,  placed  in  a  5-in.  roasting  dish,  covered 
with  another  dish,  and  roasted  in  a  muffle  with  closed  door, 
at  a  temperature  not  exceeding  a  dark  cherry-red  (635°  C.),  for 
about  35  to  40  minutes.  This  will  cause  the  volatilization  of 
95  to  96  per  cent,  of  the  antimony  as  sulphide  without  appreciable 
loss  of  gold.  The  roast  should  have  a  yellow  appearance  when 
finished,  and  can  be  fused  with  the  following  charge: 

Roasted  ore  SiO2 7  grams 

PbO 70  grams         Argol 2  grams 

Na2CO3 20  grams         Borax  glass  cover 

This  method  gives  good  results  on  jamesonite  ores. 

Arsenical  ores  are  assayed  by  the  same  methods  as  the  anti- 
monial  ores;  also  by  the  iron-nail  method,  although  this  last  is 
not  generally  to  be  recommended.  The  subject  of  the  best 
method  of  assay  of  antimonial  and  arsenical  ores  still  lacks 
thorough  investigation.  The  chief  points  may  be  outlined  as 
follows: 

1.  In   the  roasting,   unless  great   care  is  taken   as  regards 
temperature,  mechanical  loss  of  gold  and  silver  takes  place, 
owing  to  the  rapid  disengagement  of  the  arsenic  and  antimony 
oxides,  or  sulphides  of  these  metals.     Unless  the  roast  is  con- 
ducted at  a  low  heat  and  in  the  presence  of  considerable  carbon, 
arseniates  and  antimoniates  of  base  metals  or  silver  may  form, 
holding  values  which  later  on  are  not  completely  decomposed  in 
the  crucible,  owing  to  their  stability  at  a  high  temperature,  the 
result  being  appreciable  slag  losses. 

2.  In  the  niter  method,  the  presence  of  much  niter,  with  its 
powerful  oxidizing  effect,   may  also  induce  the  formation  of 
arseniates  and  antimoniates,  containing  silver  and  possibly  gold, 
which  will  remain  in  the  slag. 

3.  In  the  iron-nail  method,  unless  the  'fluxes  are  carefully 
adjusted  and  the  temperature  kept  below  1100°  C.,  speiss  carrying 
values  is  very  apt  to  form  above  the  lead  button,  and  thus  neces- 
sitate a  re-assay,  or  a  treatment  of  this  speiss. 

The  Assay  of  Arsenical  Nickel-cobalt  Silver  Ore.1 — Two  types 
of  ores  may  be  considered.  1.  Those  high  in  Ag  and  also  high 
in  Ni  and  Co  contents,  and  2.  those  low  in  Ag,  but  high  in  Ni 

i  D.  K.  Bullens,  Eng.  and  Min.  Jour.*  XC,  809.     Lodge,  Trans.  A.  I.  M.  E.,  XXX VIII, 


148  A    MANUAL    OF    FIRE    ASSAYING 

and  Co  contents.  It  is  essential  to  flux  the  Ni  and  Co  in  the  slag 
since  these  elements  seriously  interfere  with  cupellation  causing 
low  results.  Ni  present  in  the  lead  button  to  the  amount  of 
0.5  per  cent,  causes  a  scum  of  NiO  to  be  left  on  the  cupel.  More 
than  this  causes  the  "freezing"  of  the  button.  The  effect  of 
cobalt  is  not  so  pronounced  as  that  of  nickel. 

For  the  ores  high  in  silver  the  scorification  assay  is  to  be 
recommended  with  the  following  charge: 

Ore 0.05  to    0.10  a.  t. 

Lead 65  to  75        grams 

Borax  glass 3  to    5        grams 

Silica 1  to    3        grams 

Slag  and  cupel  corrections  should  be  made.  It  is  desirable  at 
times  to  check  results  by  wet  analysis  for  silver. 

For  ores  low  in  silver  the  crucible  assay  with  high  litharge 
gives  better  results  than  .the  scorification  assay.  Small  amounts 
of  ore,  0.10-0.2  a.  t.,  should  be  used,  for  the  nickel,  cobalt  and 
arsenic  in  the  ores  are  apt  to  form  a  speiss  in  the  assay.  For 
ores  containing  metallic  silver  in  any  amount  the  "scale  assay" 
should  first  be  made. 

THE  ASSAY  OF  SULPHIDES,  MAINLY  PYRITE,  BUT  CON- 
TAINING SMALL  AMOUNTS  OF  COPPER,  ZINC  SULPHIDES,  ETC. 

— Where  gold  only  has  to  be  determined  in  ores  of  this  charac- 
ter, the  roasting  method  is  satisfactory.  This,  however,  proves 
unreliable  for  silver,  and  in  many  cases  (as  at  Leadville)  the  silver 
contents  of  these  sulphides  are  the  most  important.  The  best 
method,  after  many  trials,  was  found  to  be  the  niter  fusion  on 
comparatively  small  lots  of  ore.  The  ore  has  the  following 
analysis: 

Iron 33  to  44  per  cent.         Zinc 4  to  8      per  cent. 

Sulphur 38  to  45  per  cent.         Copper 0 . 5  to  3 . 5  per  cent. 

Insoluble 4  to  20  per  cent.         Lead 0      to  0 . 4  per  cent. 

Four  assays  are  made  on  0.25  assay  ton  each,  with  3  to  4 
assay  tons  of  the  following  flux,  the  amount  depending  on  the 

reducing  power;  i.e.,  on  the  amount  of  sulphides  present: 

* 

PbO 8      parts         SiO2 1.5  parts 

KNO3 1.5  parts         Borax  glass 1.5  parts 

Na2CO3 3.0  parts 


SPECIAL    METHODS    OF    ASSAY  149 

Either  a  salt  or  a  soda  cover  is  used.  The  temperature  of 
fusion  is  brought  up  gradually  to  a  yellow  heat.  With  0.25 
assay  ton  this  gives  the  following  charge:1 

Ore 0.25  assay  ton         Na2CO3 24  grams 

PbO 62        grams  Borax  glass 11  grams 

KNO3 12        grams  SiO2 11  grams 

The  buttons  are  usually  clean,  and  separate  well  from  the  slag. 

Another  method  which  may  be  used  on  this  type  of  ore  is  the 
niter-iron  method.  This  has  the  advantage  that  no  preliminary 
assay  is  necessary  to  determine  the  amount  of  niter  for  the  proper 
size  button,  but  that  only  sufficient  niter  is  added  to  partially 
oxidize  the  sulphides,  the  iron  nails  being  relied  upon  to  decom- 
pose the  balance  of  the  ore.  On  ores  of  the  class  shown  by  the 
analysis,  the  following  charge  is  successful: 

Ore 0.5  assay  ton  SiO2 8  grams 

Na2CO3 25      grams  Borax  glass ....   8  grams 

PbO 30      grams  Iron  nails 2  to  3  tenpenny 

KNO3 15      grams  Thin  borax  glass  cover 

If  the  ore  has  a  lesser  reducing  power  than  shown  by  the 
analysis  given,  niter  and  silica  should  be  decreased  in  the  charge. 

Rapid  Methods  for  Sulphide  Ores.2 — The  approximate  per- 
centage of  sulphides  in  ores  may  be  quickly  determined  by 
vanning  with  sufficient  accuracy  for  the  addition  of  the  proper 
amount  of  niter.  An  ordinary  color  or  spotplate,  used  in  volu- 
metric chemical  analysis  is  best  used  for  the  vanning.  Small 
quantities  of  ore  are  placed  in  the  four  outside  depressions  and 
carefully  vanned  in  a  basin  of  water  until  only  the  sulphides  are 
left.  The  quantity  of  these  is  estimated  in  per  cent,  of  the  total 
amount  of  ore  taken  for  the  vanning  test.  In  gaining  experience 
with  this  method  it  may  be  desirable  for  the  assayer  to  make 
comparisons  with  ores  of  known  sulphide  contents.  If  pyrite  be 
taken  as  the  sulphide  of  unit  reducing  power,  then  chalcopyrite, 
blende,  pyrrhotite,  and  arsenopyrite  will  have  a  reducing  power 
of  §,  stibnite  of  £,  and  galena  and  chalcocite  of  £  that  of 
pyrite.  In  the  complex  sulphide  ores  the  relative  amounts  of 
the  different  sulphides  are  estimated  and  the  amounts  converted 
into  terms  of  pyrite.  In  the  ordinary  excess  litharge  charge  with  a 
fair  amount  of  borax  and  soda  and  with  0.5  a.t.  of  ore,  15  per  cent. 

1  See  also  W.  G.  Vail,  "Niter  Assay  for  Sulphide  Ores,"  in  West.  Chem.  and  Met. ,11, 14. 
2F.  G.  Hawley,  Eng.  and  Min.  Jour.,  LXXXIX,  1221,  XC,  647;  also  Min.   and  Set. 
Press,  CI,  147,  and  E.  T  Hall,  M in.  and  Sci.  Press,  CI,  345. 


150  A   MANUAL    OF    FIRE    ASSAYING 

of  pyrite  will  reduce  a  22  gram  button.  For  every  5  per  cent, 
of  pyrite  present  above  15  per  cent.,  2.1  grams  of  niter  are  neces- 
sary to  destroy  the  excess  reducing  power.  Two  stock  fluxes  are 
used  in  the  assay  of  ores.  1.  The  reducing  flux,  designed  to  give 
a  22  gram  button  with  a  neutral  ore  on  a  charge  of  0.5  a.t.  ore  and 
a  measure  or  scoop  of  flux  (84  grams).  This  flux  is  made  as 
follows:  PbO,  15  parts;  Na2C03,  4  parts;  borax,  2  parts;  flour 
0.44  parts.  When  84  grams  of  flux  are  used  this  gives  the 
following  charge: 

Ore 0.5  assay  tons 

PbO 60      grams 

Na2CO3 16      grams 

Borax 8      grams 

Flour 1.75  grams 

2.  Non-reducing  flux  to  be  used  in  connection  with  niter  for 
sulphide  ores  which  will  give  a  button  larger  than  22  grams. 
This  flux  is  made  as  follows:  PbO  15  parts;  Na2CO3,  3.5  parts; 
borax  2.5  parts;  silica  0.5  parts.  When  84  grams  of  this  flux 
are  used  it  gives  the  following  charge: 

Ore 0.5  assay  ton 

PbO 60.0  grams 

Na2CO3 14.0  grams 

Borax 10.0  grams 

Silica 2.0  grams 

Niter As  necessary 

When  sulphide  ores  are  assayed  which  do  not  contain  sufficient 
sulphides  for  a  22  gram  button,  the  reducing  and  non-reducing 
fluxes  are  mixed  in  such  proportion  as  to  obtain  the  correct  result. 
Thus — suppose  an  ore  contains  10  per  cent,  pyrite,  its  reducing 
power  would  be  ||X22  =  14.6  grams  lead  on  the  basis  of 
0.5  a.t.  The  deficiency  in  lead  is  therefore  22  —  14.6  =  7.4  grams. 
In  order  to  obtain  7.4  grams  lead  the  following  amount  of  re- 
ducing 'flux  is  required:  f f  X 7.4  =  28.27  grams.  The  balance 
of  the  charge  of  84  grams  will  be  made  up  of  non-reducing  flux, 
and  the  whole  charge  will  be:  • 

Ore 0.5    assay  ton 

Reducing  flux 28.27  grams 

Non-reducing  flux 55 . 73  grams 

The  fluxes  and  niter  are  measured  by  volume  in  properly 
designed  scoops  or  measures. 


SPECIAL   METHODS    OF   ASSAY  151 

For  high  sulphide  ores  when  very  accurate  results  are  required 
a  preliminary  assay  is  made  as  follows: 

Ore 3 . 64  grams 

Non-reducing  flux 50.0    grams 

This  is  run  in  a  10  gram  crucible.  This  charge  will  give  a 
lead  button  weighing  as  much  as  the  niter  necessary  to  oxidize 
all  the  sulphides  in  0.5  a.t.  of  the  ore.  Place  the  lead  button 
obtained  in  one  scale  pan  of  the  pulp  scale  and  from  the  hook 
above  the  other  pan  suspend  by  fine  wire  a  weight  so  that  with  the 
wire  it  amounts  to  6  grams.  Then  add  niter  to  the  pan  having  the 
6  gram  weight  until  the  scale  is  in  balance.  This  amount  of 
niter  is  the  proper  amount  necessary  to  produce  a  22  gram  button 
with  the  ore  and  the  non-reducing  flux  if  0.5  a.t.  of  ore  is  taken 
for  assay.  (Consult  Chapter  V.) 

For  important  assays  it  is  desirable  to  make  4  assays,  combine 
the  buttons  from  2,  and  scorify  into  one  button  each.  Make 
the  two  cupellations,  weigh  the  beads  separately  for  Ag,  combine 
them  for  parting  and  make  one  weighing  on  gold. 

If  the  ores  assayed  contain  more  than  12  per  cent,  copper,  it  is 
desirable  to  take  the  lead  buttons  from  the  assay  and  place  them 
into  crucibles  with  50  grams  of  litharge  and  2  grams  SiO2,  place 
in  the  muffle  and  leave  there  four  or  five  minutes  after  the  PbO 
has  melted.  Then  withdraw  the  crucible  and  with  the  tongs 
give  the  contents  a  rapid  swirling  motion  for  a  few  minutes  and 
then  pour.  This  treatment  eliminates  most  of  the  copper  re- 
maining in  the  button.  Then  cupel  and  part  as  usual.  It  is  to 
be  noted  that  the  methods  described  may  have  to  be  modified 
to  suit  particular  conditions. 

THE  ASSAY  OF  MATERIAL  CONTAINING  METALLIC  SCALES. 

— Ores  of  this  kind  are  difficult  to  assay  and  obtain  correct  re- 
sults from,  as  the  metallic  particles  (usually  gold  or  silver)  are  so 
unevenly  distributed  as  to  make  it  practically  impossible  to  ob- 
tain an  accurate  sample.  Two  methods  of  assay  are  available: 
(a)  Approximately  500  grams  of  ore  (or  less,  if  deemed  advis- 
able) are  weighed  out,  crushed,  and  put  through  a  150-  or  200- 
mesh  screen,  care  being  taken  to  separate  out  the  scales  as  closely 
as  possible.  Screening  and  crushing  should  frequently  succeed 
each  other.  When  all  the  scales  have  been  separated  out,  they 
are  transferred  to  a  parting  cup  and  dissolved  in  3  to  5  c.c.  of 
nitro-hydrochloric  acid,  if  gold,  or  in  nitric  acid  if  silver  or  copper. 
The  pulp  is  then  heaped  up  into  a  cone  in  a  large  porcelain  dish, 


152  A    MANUAL    OF    FIRE   ASSAYING 

the  gold,  etc.,  solution  poured  on  the  apex  of  the  cone,  and  the 
parting  cup  washed  out  thoroughly  with  warm  distilled  water, 
using  no  more  than  is  necessary  to  completely  wash  it  out.  The 
bed  of  pulp  should  be  thick  enough  to  readily  absorb  all  of  the 
solution,  and  not  permit  it  to  penetrate  to  the  dish.  The  pulp  is 
then  dried  in  an  air  bath  at  120°  C.,  thoroughly  mixed  on  glazed 
paper,  and  put  through  the  screen  repeatedly.  It  is  then  assayed 
by  the  method  suitable  to  it,  like  any  other  ore. 

(6)  From  200  to  500  grams  of  ore  are  weighed  out,  crushed 
and  screened,  and  the  scales  separated,  as  described  above.  The 
scales  and  the  pulp  are  then  weighed  and  the  loss  in  dusting 
noted.  The  scales  are  assayed  by  scorification;  the  lead  button 
is  cupelled,  and  the  bead  weighed  and  parted.  Then  15  grams 
of  the  ore  is  weighed  out  in  duplicate,  fused  with  the  proper 
charge,  the  lead  buttons  from  these  fusions  cupelled,  and  the 
beads  weighed  and  parted.  From  the  results  obtained,  the  total 
amount  of  gold  and  silver  in  the  original  ore  is  calculated,  con- 
sidering both  pulp  and  scales.  The  gold  and  silver,  respectively, 
found  is  multiplied  by  29.166  and  divided  by  the  original  weight 
of  ore,  taken  in  grams;  this  gives  the  value  in  ounces  per  ton. 

THE  ASSAY  OF  ORES  CONTAINING  THEIR  CHIEF  VALUE 

IN  FREE  GOLD. — As  already  pointed  out,  these  ores  are  difficult 
to  get  correct  results  from.  Even  though  the  free  gold  particles 
are  very  fine,  it  is  impossible  to  distribute  them  uniformly 
throughout  the  bulk  of  the  sample.  The  proper  way  to  assay 
material  of  this  kind  is  to  take  from  1000  to  1500  grams  of 
the  ore,  crushed  through  a  100-mesh  screen,  place  in  a 
large  Mason  jar  with  a  tight  screw  cover,  mix  to  a  rather  thick 
pulp  with  water  and  then  add  3  to  4  c.c.  of  mercury  from 
a  burette.  It  is  essential  that  the  mercury  should  be  free 
from  gold  and  silver,  or  its  contents  of  precious  metals 
known.  Most  mercury  as  purchased  contains  some  gold.  The 
jar  and  its  contents  are  then  agitated  for  two  hours,  best  in 
some  mechanical  agitator.  Then  carefully  separate  the  mercury 
from  the  ore  by  panning  in  a  gold  pan,  saving  all  the  pulp,  in 
another  pan  of  somewhat  larger  size.  None  of  the  fine  slimes  of 
the  ore  must  be  permitted  to  escape.  It  may  be  necessary  to  add 
a  little  more  mercury  and  a  very  little  sodium  amalgam  during 
panning  to  collect  any  floured  and  sickened  mercury.  The  pulp 
is  allowed  to  settle  in  the  pan,  the  surplus  water  carefully  poured 
off,  and  the  pan  then  set  on  a  hot  plate  to  dry.  When  dry  it 


SPECIAL    METHODS    OF    ASSAY  153 

is  mixed  on  a  cloth,  and  1  a.t.  samples  taken  and  assayed  by  a 
proper  method.  The  mercury  is  carefully  transferred  from  the 
pan  to  a  porcelain  dish,  washed  with  water  to  free  from  sands, 
dried  with  filter-  or  blotting-paper  and  then  transferred  to  a 
20  gram  crucible  in  which  20  grams  of  lead  have  been  placed.  To 
the  crucible  is  then  added  a  charge  consisting  of  30  grams  PbO, 
10  grams  Na2C03,  5  grams  borax  glass  and  0.5  gram  argol 
and  silver  foil  enough  to  part  the  gold. 

The  fusion  is  made  by  raising  the  heat  very  gradually;  it  is  best 
to  use  a  muffle  that  has  not  yet  become  red,  and  has  a  good 
draft  through  it,  to  prevent  the  escape  of  mercury  fumes  into 
the  room.  The  button  from  the  fusion  is  cupelled  in  the  usual 
manner.  The  gold  is  weighed  in  mgs.  and  the  weight  divided 
by  the  grams  of  ore  taken  and  multiplied  by  29.166}  gives  the 
oz.  gold  per  ton  present  as  "free"  gold.  This  figure  added  to 
the  assay  results  from  the  pulp  gives  the  total  contents  of  the 
ore  in  oz.  per  ton. 

Another  method1  is  carried  out  as  follows: 

Take  6  a.t.  of  the  sample  crushed  to  pass  80  mesh,  add  sufficient 
water  and  a  small  amount  of  sulphuric  acid  to  make  a  thin 
paste  in  an  8  in.  porcelain  mortar,  add  8  grams  of  redistilled 
mercury  and  grind  thoroughly  for  30  minutes.  Then  separate 
the  tailings  from  the  mercury  by  washing  them  off  with  a  stream 
of  water  obtained,  say  by  attaching  a  hose  to  a  hydrant.  During 
the  washing  the  mortar  should  be  given  a  rotary  motion.  Collect 
the  overflow  from  the  mortar  in  a  large  gold  pan  and  treat  the 
pannings  as  described  in  the  method  above.  When  the  mercury 
in  the  mortar  is  quite  clean  from  sands  give  it  a  final  wash  with 
water,  then  dry  it  with  filter-paper  and  transfer  to  a  crucible 
containing  enough  litharge  with  reducing  agent  to  giye  a  20 
grams  button.  Add  enough  silver  to  part  the  gold  and  start  the 
fusion  at  a  very  low  heat  in  the  furnace.  Cupel  the  lead  button 
and  weigh. 

Divide  the  weight  of  the  gold  by  the  number  of  assay  tons  of 
ore  taken  and  add  this  to  the  figure  obtained  from  the  assay  of 
the  pulp  in  order  to  get  the  total  value  of  the  ore  in  ounces  per  ton. 
When  the  ore  to  be  assayed  contains  arsenopyrite  and  graphite 
some  of  these  will  adhere  to  the  mercury.  In  order  to  overcome 
this  difficulty  add  to  the  washed  mercury  in  the  porcelain  mortar 
5  c.c.  of  cone.  HNO3  and  enough  silica  to  make  a  thin  paste. 

>  A.  T.  Roos,  Mining  World,  XXXII,  319. 


154  A   MANUAL    OF    FIRE   ASSAYING 

Grind  for  a  few  minutes  and  then  wash  the  silica  and  acid  off 
with  water  and  proceed  as  before. 

AMALGAMATION  TEST  TO  DETERMINE  THE  AMOUNT  OF 

"FREE"  GOLD  PRESENT.1— One  hundred  grams  of  crushed 
ore  are  weighed  out  into  a  citrate  of  magnesia  bottle,  150 
c.c.  of  water  added  and  then  2  c.c.  of  pure  mercury  from 
a  burette.  The  stopper  is  clamped,  the  bottle  rolled  in  a 
piece  of  cloth  and  placed  in  a  moving  shaker  for  two  hours. 
It  is  then  removed,  opened,  covered  with  the  thumb,  shaken, 
and  inverted  over  a  3  in.  porcelain  dish,  and  as  much  clean 
mercury  as  possible  allowed  to  run  out.  A  little  more  water 
is  added  and  more  mercury  allowed  to  run  out  into  another 
dish  and  so  on  as  long  as  any  comes  out.  If  the  mercury 
is  not  floured  nearly  all  is  removed  in  two  operations.  All 
the  clean  mercury  is  then  put  into  a  250  c.c.  beaker.  The 
bottle  is  then  shaken  well  and  again  inverted  to  let  a  little 
sand  run  out  into  a  dish.  This  sand  is  then  panned  into  an 
enameled  dish,  usually  only  a  few  globules  of  mercury  being 
obtained.  If  much  is  found  from  this  third  inversion  the  whole 
charge  must  be  panned  and  if  the  mercury  is  floured  a  small  glob- 
ule of  liquid  sodium  amalgam  should  be  added  to  the  pan. 
In  ordinary  routine  work  the  tailings  from  the  panning  are 
discarded.  For  special  purposes  as  when  the  tailings  are  to  be 
tested  by  cyaniding,  concentration,  etc.,  they  are  allowed  to 
settle  completely,  decanted  and  if  necessary  dried  for  further 
tests  or  for  assay.  To  the  mercury  after  its  collection  in  the 
250  c.c.  beaker  0.5  gram  of  pure  silver  is  added  (if  this  has  been 
carefully  prepared  and  cleaned  by  treating  in  cyanide  solution  or 
weak  nitric  acid  or  by  slightly  amalgamating  the  surface  it 
may  be  used  to  pick  up  the  small  globules  of  mercury  collected 
in  panning)  about  150  c.c.  of  HN03,  sp.  gr.  1.14,  previously 
warmed  to  about  70°  C.  is  now  poured  into  the  beaker  which  is 
set  into  an  enameled  pan  on  a  hot  plate  and  left  there  till  all 
the  mercury  has  dissolved.  If  not  too  hot  it  is  unnecessary  to 
cover  the  beaker.  As  soon  as  the  mercury  disappears  the  liquid 
is  filtered  on  a  12.5  cm.  paper  previously  wetted.  The  residue 
is  rinsed  on  to  the  paper,  washed  once  or  twice  with  very  dil.  HNO3 
(not  over  5  per  cent.)  and  once  with  water.  Test  lead  is  then 
sprinkled  on  the  paper,  it  is  folded,  placed  on  test  lead  in  a 
scorifier  and  enough  lead  added  to  make  a  20  gram  button. 

1  Method  used  at  the  Homestake  Mine,  S.  D.     Communicated  by  Wm.  J.  Sharwood. 


SPECIAL    METHODS    OF   ASSAY 


155 


Silver  is  added  to  insure  parting  and  also  a  few  grams  of  borax 
glass.  The  scorifier  is  then  charged  into  the  muffle,  the  paper 
burned,  the  charge  scorified  for  a  few  minutes,  poured,  the 
button  cupelled  and  the  bead  parted  and  the  gold  weighed. 
From  a  100  gram  ore  sample  each  mg.  of  gold  represents  0.29166 
oz.  or  $6.03  free  gold  per  ton. 

Notes  on  the  Carrying  Out  of  the  Amalgamation  Test. — The  amal- 
gamation test  is  carried  out  for  the  purpose  of  determining  the 
amount  of  precious  metals  that  can  be  recovered  from  the  ore 
in  milling  operations  by  means  of  amalgamation.  The  size  of  the 
crushed  ore  will  influence  the  results;  therefore,  in  different  tests 
the  degree  of  fineness  must  be  nearly  constant.  The  exact  fine- 
ness used  depends  upon  conditions.  Temperature  has  its  influ- 


FlO.   57. HOMESTAKE   AalTATOB  FOR   AMALGAMATION  AND  CYANIDE  TESTS. 

ence;  if  the  tests  are  carried  out  at  temperatures  higher  than  the 
normal  daily  temperature  results  will  be  higher.  If  the  tempera- 
ture be  low  results  will  be  lower.  The  addition  of  silver  to  the 
mercury  reduces  the  time  required  for  its  solution  by  about  one- 
half.  Extreme  care  must  be  taken  to  get  mercury  and  silver 
practically  free  from  gold.  It  is  desirable  to  run  a  blank  assay 
on  say  20  c.c.  of  mercury  and  5  grams  of  silver  (representing  ten 
times  the  quantity  of  each  used  in  the  assay).  The  mercury  and 
silver  are  dissolved  in  acid  as  stated  in  the  amalgamation  test 
and  the  residues  treated  as  described.  The  best  mercury  obtained 
after  testing  a  number  of  new  flasks  and  when  used  with  commer- 
cial proof  silver  required  a  correction  of  0.02  mg.  of  gold  for  2  c.c. 
mercury  and  0.5  gram  silver.  The  silver  contained  nearly  half 
of  the  gold  thus  found.  If  the  amount  of  silver  in  the  ore 
recoverable  by  amalgamation  is  to  be  determined  the  parting 
of  the  mercury  by  nitric  acid  must  be  replaced  by  the  crucible 


156  A   MANUAL    OF    FIRE    ASSAYING 

fusion  of  the  mercury  as  described  in  method  1  for  the  assay  of 
ores  containing  free  gold. 

Equipment  Required  for  Routine  Amalgamation  Tests. — 
1.  Shaking  box  with  24  compartments  each  3  in.  square  and 
6  in.  deep  as  shown  in  Fig.  57.  The  following  are  the  details 
of  construction.  Sides  and  bottom  made  of  1.75  in.  lumber; 
partitions  of  0.5  in.  lumber;  sills  4X4  in.  lumber;  connecting  rod 
If  Xf  in.  oak  lumber,  4  ft.  long;  supports  of  light  steel  l.SX-j^- 
in.  and  2  ft.  long;  shaft  1  in.  250  r.  p.  m.;  throw  of  eccentric, 
2  in.  A  frue  vanner  eccentric  rod  and  supports  may  be  used  in 
the  construction  of  the  agitator.  A  pad  should  be  placed  at  the 
bottom  of  each  pocket.  Pieces  of  canton  flannel,  12  to  15  in. 
square,  are  used  to  wrap  each  magnesia  bottle. 

2.  Twenty-four  citrate  of  magnesia  bottles  with  spring  clamps 
and  rubber  washers.     These  have  a  capacity  of  350-370  c.c. 

3.  Twenty-four  beakers,  capacity  about  250  c.c. 

4.  Two  enameled  iron  pans  to  hold  12  beakers  each. 

5.  Six  porcelain  dishes,  3  in.  in  diameter  and  2  enameled  iron 
pans,  8  in.  in  diameter  and  2  in.  deep. 

6.  Filtering  rack  for  12-2£  in.  funnels.     Twelve  extra  beakers. 

7.  Copper  weighing  scoop  and  copper  funnel  with  steep  sides 
for  charging  bottles. 

8.  Cylinder  graduated  to  100,  150,  and  200  c.c. 

9.  Glass  stopper  burette  standing  in  enameled  iron  pan. 
10.  Supplies  as  mentioned  in  the  assay. 

THE  ASSAY  OF  CYANIDE  SOLUTIONS.    Method  I.1— Measure 

out  any  convenient  volume  into  a  beaker  (preferably  10  or  20  a.t 
using  beakers  of  500  to  700  c.c.  capacity).  Add  10  to  20  c.c.  of 
lead  acetate  solution  containing  10  to  20  per  cent,  of  the  salt,  then 
introduce  3  to  4  grams  of  zinc  dust  in  the  form  of  an  emulsion  or 
suspension  in  water  and  stand  on  a  hot  plate.  When  moderately 
heated  but  before  boiling,  acidify  with  about  20  c.c.  strong 
hydrochloric  acid,  either  c.p.  or  of  the  best  commercial  grade. 
Boil  until  action  nearly  ceases,  and  the  reduced  lead  has  collected 
into  a  spongy  mass.  Filter  on  a  "quick"  paper,  and  wash  pre- 
cipitate twice  with  hydrant  water.  Remove  the  filter-paper  and 
precipitate  and  squeeze  out  as  much  water  as  possible.  Place 
in  a  2  in.  scorifier  with  10  to  15  grams  of  test  lead  and  3  to  5 
grams  of  borax  glass.  Place  at  once  in  the  muffle,  burn  the  paper, 
scorify  for  only'a  few  minutes,  pour,  cupel  lead  button,  part  and 

'  Due  to  Mr.  Allan  J.  Clark,  Homeetake  Mining  Co. 


SPECIAL   METHODS    OF   ASSAY  157 

weigh.  Unless  silver  is  to  be  determined,  silver  foil  should  be 
added  to  the  scorifier  for  inquartation,  or  a  measured  volume  of 
dilute  AgN03  solution  may  be  added  to  the  beakers  from  a 
burette. 

Notes  on  the  Method. — About  100  grams  zinc  dust  are  usually 
mixed  with  300  c.c.  of  water  in  a  bottle  with  an  £-in.  glass  tube 
passing  through  the  cork.  This  mixture  is  shaken  into  a  capsule 
of  the  proper  size  used  as  a  measure.  The  other  reagents  must  be 
roughly  measured.  Their  proportions  should  be  varied  slightly 
until  conditions  are  found  which  yield  a  "  sponge  "  of  lead  quickly 
with  the  particular  solutions  regularly  assayed.  Impure  hydro- 
chloric acid  does  not  give  good  results,  nor  do  other  acids.  It 
is  essential  that  nearly  all  the  zinc  be  dissolved  before  filtering. 
Comparatively  cheap  filter-papers  answer  well.  If  a  300  c.c. 
flask  be  regraduated  to  deliver  301.45  c.c.  every  mg.  of  gold 
obtained  from  this  volume  represents  $2.00  gold  value  per  ton. 
If  copper  is  present  in  solutions  a  somewhat  longer  scorification 
than  above  stated  may  be  desirable. 

The  method  was  suggested  by  Chiddey's  method1  in  which 
zinc  shavings  and  lead  salt  are  used  to  produce  a  lead  sponge. 
In  this  original  method  the  lead  sponge  is  recovered  by  hand  and 
not  filtered  and  then  cupelled  direct  without  a  preliminary 
scorification. 

Clark's  method  gives  somewhat  better  results  on  low  grade 
solutions  than  evaporation  methods  with  litharge  or  litharge 
bearing  flux.  It  has  the  particular  advantage  of  being  an  ex- 
ceedingly rapid  method  as  compared  to  the  tedious  evaporation 
methods. 

Method  2.  Evaporation  Method. — Measure  out  5  to  10  a.  t. 
or  more  of  solution  by  means  of  a  properly  graduated  flask  and 
transfer  to  either  porcelain  or  agate  ware  evaporating  dishes,  of 
300  to  500  c.c.  capacity.  To  the  solution  add  50  to  60  grams  of 
litharge  and  place  the  dishes  in  a  sand  bath  on  a  hot  plate  and 
carefully  evaporate  to  almost  complete  dryness.  If  agate  wrare 
dishes  are  used  it  is  essential  that  the  agate  lining  be  unbroken, 
otherwise  precious  metals  will  precipitate  on  the  iron  surface  and 
adhere  to  the  same,  giving  low  results  in  the  assay.  Tin  dishes 
should  not  be  used.  When  practically  dry  transfer  contents  by 

1  A.  Chiddey,  Eng.  and  Min.  Jour.,  LXXV,  473.  Consult  also  W.  H.  Barton.  West. 
Chem.  and  Met.,  IV,  67,  and  A.  Whitby,  Jour.  Chem.  Met.  and  Min.  Soc.  S.  A.,  X, 
134,  211,  288. 


158  A   MANUAL    OF    FIRE   ASSAYING 

means  of  a  spatula  to  a  glazed  paper,  and  remove  any  adhering 
litharge  from  the  dish  by  means  of  a  moist  piece  of  filter-paper, 
thoroughly  wiping  out  the  dish.  If  the  evaporation  has  not  been 
carried  too  far  this  can  readily  be  done.  Then  mix  in  a  20  grams 
crucible,  25  grams  litharge,  15  grams  Na2CO3,  2  grams  argol,  2 
grams  SiO2  and  5  grams  borax  glass,  and  transfer  the  litharge  from 
the  evaporation  and  filter-paper  to  the  crucible  and  again  mix 
with  a  spatula.  Fuse  the  charge  and  proceed  as  usual.  Unless 
silver  is  to  be  determined  add  silver  foil  to  the  crucible  before 
fusion. 

Evaporation  methods  conducted  in  dishes  made  of  lead  foil 
have  the  disadvantage  of  permitting  the  use  of  comparatively 
small  quantities  of  solution  only  and  very  frequently  give  low 
results. 

THE  ASSAY  OF  SLAGS  AND  CUPELS  FOR  THE  CORRECTION 
ASSAY. — (a)  Slags:  The  charge  for  these  depends  upon  whether 
they  are  acid  or  basic.  Particular  care  must  be  taken  to  get  a 
charge  that  will  completely  decompose  the  original  slag.  If  this 
is  acid,  the  charge  should  aim  to  make  a  new  slag  more  basic, 
and  vice  versa.  The  lead  button  should  be  from  25  to  30  grams 
in  weight.  Many  assayers  frequently  add  simply  litharge  and 
reducing  agent  to  the  slag  in  making  the  fusion.  This  is  not 
always  desirable,  for  if  the  slag  already  has  much  litharge  in  it, 
soda,  etc.,  may  with  profit  be  added  as  the  extra  base  in  place 
of  litharge. 

(6)  Cupels:  The  bone- ash  of  the  cupel  will  not  unite  with 
fluxes  to  form  slags,  but  remains  suspended  in  the  fusion.  For 
this  reason  the  cupel  should  be  put  through  a  150-  to  200-mesh 
screen  before  assaying,  the  litharge-stained  portion  only  being 
taken.  For  one  large  cupel,  or  two  small  ones,  the  charge  is  as 

follows: 

* 

Cupel  Borax  glass 45      grams 

PbO 60  grams         Argol 2.5  grams 

Na2CO, 25  grams         Soda  cover 

Fluorspar  is  not  desirable  in  the  assay  of  cupels,  as  it  merely 
adds  another  ingredient  in  suspension. 

Magnesia  cupels  may  be  fluxed  with  the  following  charge: 

Cupel                                                             Borax  glass 20      grams 

PbO 40  grams         Silica 10      grams 

NaaCOa • 20  grams         Argol 2.5  grams 

Borax  cover 


SPECIAL   METHODS    OF    ASSAY  159 

Cement  cupels  are  more  easily  fluxed  and  an  ordinary  crucible 
charge  for  a  somewhat  basic  ore  will  answer  very  well. 

THE  ASSAY  OF  MATERIAL  CONTAINING  METALLIC  IRON.1— 
Material  of  this  kind  will  be  obtained  in  the  clean  up  of  mortar 
boxes  of  stamp  mills,  the  iron  being  present  as  pellets,  and  much 
larger  pieces  mixed  with  sand,  pebbles,  etc.  It  cannot  be  crushed 
and  is  assayed  in  the  state  received.  Its  correct  sampling  is 
practically  impossible.  Crucible  fusions  are  made  in  the  pres- 
ence of  bisulphate  of  soda  and  niter.  The  charge  is  as  follows: 

Material  to  be  assayed 1  assay  ton 

Bisulphate  of  soda 8  to  24  grams 

Na2CO3 25  grams 

SiO2 10  grams 

Borax  glass 25  grams 

Litharge 35  grams 

Niter 1  to  4  grams 

The  fusion  should  be  conducted  at  a  high  heat  for  about  45 
minutes.  Then  add  to  the  crucible  15  grams  of  PbO  mixed  with 
2  grams  argol  and  continue  fusion  for  20  min.  more  until  quiet. 

The  action  of  the  bisulphate  is  probably  as  follows.  It  breaks 
up  on  heating. 

2NaHS04  =  Na2S04  +  H20  +  SO3 

The  metallic  iron  is  converted  into  FeSO4  by  the  SO3  in  the  early 
stage  of  the  fusion,  and  is  then  converted  into  ferrous  silicate  as  the 
temperature  rises.  The  litharge  and  niter  aid  in  the  oxidation  of 
the  iron.  Practically  all  of  the  PbO  is  reduced  by  the  metallic 
iron.  Interaction  also  takes  place  between  the  NaHSO4  and  the 
Na2CO3  dependent  on  the  quantities  present.  Na2CO3  may  with 
advantage  be  replaced  by  lime  for  this  reason.  The  charge  may 
have  to  be  modified  considerably  in  quantities  of  the  reagents 
present  to  suit  the  material  to  be  assayed. 

i  "Modification  of  Method  of  H.  R.  Jolly,"  Jour.  Chem.  Met.  and  Min.  Soc.  S.  A.,  VIII. 
343. 


CHAPTER  XI 
ERRORS  IN  THE  ASSAY  FOR  GOLD  AND  SILVER 

LOSSES  IN  THE  CUPELLATION  OF  PURE  GOLD  AND  SILVER.— 

These  losses  may  be  divided  into  (1)  losses  by  absorption,  (2)  losses 
by  volatilization.  The  losses  of  gold  and  silver  in  the  cupellation 
are  functions  of  (a)  the  temperature  of  cupellation;  (b)  the  amount 
of  lead  with  which  the  gold  and  silver  is  cupelled;  (c)  the  physical 
nature  of  the  cupel;  (d)  the  nature  and  amount  of  impurities 
present;  (e)  the  influence  which  silver  has  on  the  gold  loss,  and 
vice  versa. 

There  is  considerable  literature  extant  upon  losses  in  cupella- 
tion of  the  two  precious  metals,  but  in  the  older  researches  the 
temperature  influence  is  but  vaguely  defined,  owing  to  the  lack 
of  means  for  ready  and  satisfactory  temperature  measurements, 
a  deficiency  which  is  now  supplied  by  the  LeChatelier  platinum- 
rhodium  pyrometer.  Losses  are  also  expressed  as  percentages  of 
the  total  amount  of  metal  cupelled,  and  then  the  average  per- 
centage losses  are  indicated.  That  this  is  very  deceptive  is  made 
evident  by  reference  to  the  curve  of  losses  accompanying  this 
chapter. 

It  is  for  this  reason  that  the  statement  of  results  given  by 
Mason  and  Bowman,1  that  the  average  loss  in  cupellation  of  pure 
silver  under  normal  conditions  is  1.99  per  cent,  and  for  gold 
0.296  per  cent.,  does  not  convey  any  very  definite  idea,  unless  the 
amount  of  metal  cupelled  is  accurately  specified,  as  well  as  the 
temperature.  This  fact  has  been  noted  by  other  observers,2  but 
no  effort  has  been  made  to  express  results  coordinately. 

The  following  data  show  the  losses  which  occur: 

1  Jour.  Am.  Chem.  Soc.,  XVI,  505. 

2  Kaufman,  in  Eng.  and  Min.   Jour.,  LXXIII,  829.      Miller  and  Fulton,  in   "School  of 
Mines  Quart.,"  XVII,  169 


ERRORS    IN    THE    ASSAY    FOR    GOLD    AND    SILVER 


161 


TABLE  XXVIII.— CUPELLATION  OF  PURE  SILVER 
(J.  EAGER  AND  W.  WELCH  x) 


Amt.  of  silver 
milligrams 

Amt.  of  lead 
grams 

Temperature 
deg.  Cent.2 

Total  losses 
per  cent. 

204.62 

10 

700 

1  .  02  (average) 

205 

10 

775 

1.28 

203 

10 

850 

1.73 

203 

10 

925 

3.65 

203 

10 

1000 

4.87 

TABLE  XXIX.— CUPELLATION  OF  PURE  SILVER 

(L.    D.    GODSHALL3) 


Amt.  of  silver      .     Amt.  of  lead 
Mgs.                           grams 

Approximate 
Temp.  deg.  Cent, 
of  air  in  muffle 

Total  loss  in 
per  cent. 

2 

7.5 

750° 

3.66 

2 

15.0 

750 

4.40 

2 

22.5 

750 

5.52 

2 

30.0 

750 

5.96 

5 

7.5 

750 

3.29 

5 

15.0 

750 

2.63 

5 

22.5 

750 

3.83 

5 

30.0 

750 

4.31 

10 

7.5 

750 

3.73 

10 

15.0 

750 

2.89 

10 

22.5 

750 

4.47 

10 

30.0 

750 

4.26 

20 

7.5 

750 

3.42 

20 

15.0 

750. 

2.34 

20 

22.5 

750 

3.59 

20 

30.0 

•750 

3.10 

50 

7.5 

750 

2.14 

50 

15.0 

750 

2.46 

50 

22  .  5                          750 

2.33 

50 

30.0 

750 

2.89 

100 

7.5 

750 

2.11 

100 

15.0 

750 

2.40 

100 

22.5 

750 

2.10 

100 

30.0 

750 

2.28 

200 

7.5 

750 

1.71 

200 

15.0 

750 

1.64 

200 

22.5 

750 

1.62 

200 

30.0 

750 

2.07 

1  Lodge,  "Notes  on  Assaying,"  p.  59. 

2  Of  air  in  muffle,  directly  above  cupel. 

3  Trans.  A.  I.  M.  E.,  XXVI,  473. 

11 


162 


A   MANUAL    OF    FIRE    ASSAYING 


TABLE  XXX.— CUPELLATION  OF  PURE  SILVER. 

(KAUFMAN1) 


Amt.  of  silver 
Mgs. 


Amt.  of  lead 
grams 


Approximate     | 

Temp.  deg.  Cent.  I 

of  air  in  muffle    ; 


Total  loss  in 
per  cent. 


25 

5 

750° 

2.14 

25 

10 

750 

2.63  (2.38,  2.43) 

25 

15 

750 

2.69 

25 

25 

750 

2.09  (2.48,  2.44) 

50 

5 

750 

1.43 

50 

10 

750 

2.23  (2.10,  1.96) 

50 

15 

750 

2.14 

50 

25 

750 

.86  (2.25,  2.37) 

100 

5 

750 

.30 

100 

10 

750 

.61  (1.82,  1.42) 

100 

15 

750 

.68 

100 

25 

750 

.12  (1.93,  2.12) 

200 

5 

750 

.86 

200 

10 

750 

.24  (1.29,  1.17) 

200 

15 

750 

.40 

200 

25 

750 

.74  (1.46,  1.76) 

Parentheses  indicate  different  types  of  cupels,  viz.,  bone-ash 
made  up  respectively  with  pearl-ash  and  stale  beer.  The  main 
figures  were  obtained  by  bone-ash  oupels  made  up  with  water. 
The  results,  viewed  as  a  whole,  indicate  that  all  three  types  have 
equal  merit.  Godshall  (Table  XXIX),  experimented  with 
different  types  of  standard  bone-ash  cupels  (some  made  at  the 
mint),  with  the  same  result. 

The  agreement  amongst  the  different  writers  is  very  close, 
when  the  fact  is  taken  into  consideration  that  in  the  last  two 
cases  no  precise  statement  concerning  temperature  is  made,  and 
that  the  amounts  of  lead  differ  somewhat. 


1                              ! 

Authority 

Amt.  of 
silver  mgs. 

Amt.  of  lead 
Grams 

Temperature 
Deg.  Cent. 
Air  in  muffle. 

Total  loss 
per  cent. 

Eager  and  Welch. 

205 

10 

775 

1.28 

Godshall  200 

7.5 

?(feathers) 

1.71 

Kaufman  

200 

10 

?  (feathers) 

1.24 

Liddell2  

102 

20 

?(feathers) 

1.70 

'  Eng.  and  Min.  Jour.,  LXXIII,  829. 


2  Eng.  and  Min.  Jour.,  LXXXIX,  1264. 


ERRORS    IN    THE   ASSAY   FOR   GOLD    AND    SILVER 


163 


1+       '                     i- 

:    + 

1       »' 

°   i! 

1 

| 

!  Ij 

j     / 

I 

JS      '•  + 

cj           :      i 

e     !  + 

a          :      I 

S      i  | 

/      ' 

•s     i  i 

|j 

1       11 

i 

j 

• 

'•§  '    i    | 

0 

i 

| 

.    / 

!  |t 

sf 

•    ! 

)/ 

§::: 

1 

1 
/ 

Jj 

1 

g    -1    -Hi.*.-! 

"i! 

I 

I'       :       J 

/, 

// 

i 

/  i 

/     I 

/  ,- 

/] 

f/j 

(/ 

i- 

/// 

/ 

/  /  / 

/I 

/ 

i 

/  i 

'    !     \ 

8- 

///I 

6601  JO 


164 


A    MANUAL    OF    FIRE    ASSAYING 


The  accompanying  curves  are  constructed  from  figures  in  Mr. 
GodshalPs  paper.  The  general  averages  are  taken,  and  while 
his  losses  are  perhaps  a  trifle  higher  than  the  best  work  calls 
for  at  the  present  day  (owing  to  a  better  recognition  of  the  precise 
temperature  required),  they  form  the  best  and  most  complete 
data  for  the  construction  of  curves  showing  the  relation  between 
amounts  of  silver  cupelled  and  the  percentage  loss.  I  refrain 
from  a  mathematical  discussion,  but  an  equation  covering  the 
case  is  tentatively  offered.1 

The  influence  of  the  size  of  lead  button  is  clearly  discernible 
by  the  ordinates  of  the  curves.  The  temperature  variations  will 
show  in  the  same  way. 

The  literature  of  gold  losses  is  considerably  less  than  that  for 
silver.  Rose2  discusses  them  in  the  gold  bullion  assay.  He 
gives  the  total  loss  on  bullion  916.6  fine,  under  normal  tempera- 
ture conditions,  as  from  0.4  to  0.8  per  1000,  of  which  82  per  cent, 
is  cupel  absorption,  10  per  cent,  volatilization  (probably),  and 
8  per  cent,  solution  in  acid.  This,  calculated  to  percentage  on 
actual  gold,  is  equivalent  to  0.0803  per  cent,  for  the  highest  loss. 
(This  is  cupel  loss  only,  not  including  solution  loss.) 

Hillebrand  and  Allen's  results  contain  interesting  data  re- 
garding the  relative  losses  by  absorption  and  volatilization,  to 
which  reference  will  be  made  again. 

CUPELLATION  OF  GOLD-SILVER  ALLOYS.— The  loss  of  gold 
and  silver  in  cupellation  is  somewhat  different  when  both  gold 
and  silver  are  present  from  the  loss  when  either  metal  alone 
is  present. 

TABLE  XXXI.— CUPELLATON  OF  GOLD 
(EAGER  AND  WELCH3) 


Amt.  of  gold 


Amt.  of  lead     i     Temp.  °C.4    ,     Total  loss  per  cent. 


201 

10 

775° 

0.155 

201 

10 

850 

0.430 

204 

10 

925 

0.460 

201 

10  . 

1000 

1.430 

201 

10 

1075 

3.000 

1 1  am  indebted  to  Prof  C.  C.  Van  Nuys, 
!  "  Metallurgy  of  Gold,"  1902,  p.  506. 
3  Lodge,  "Notes  on  Assaying,"  p.  142. 
*  Of  air  in  muffle,  directly  above  cupel. 


I.  A.,  for  the  curves  and  the  equations. 


ERRORS    IN    THE    ASSAY    FOR    GOLD    AND    SILVER 


165 


TABLE  XXXII.— CUPELLATION  OF  GOLD 

(HlLLEBRAND    AND    ALLEN1) 


Amt.  of 
gold 
mgs. 

Amt.  of 
lead 
grams 

Approximate 
Temp.  deg.  Cent, 
of  air  in  muffle 

Total  loss 
per  cent. 

Total  loss 
cupel 
absorption 
per  cent. 

Total  loss 
volatilized 
per  cent. 

30  58 

25 

750°  feathers 

0  36 

30.32 
30.63 
30.45 
30.16 
30.66 
10  34 

25 
25 
25 
25 
25 
25 

increased 
increased 
increased 
increased 
back  of  muffle 
750°     front     of 

1.19 
1.76 
3.78 
4.17 
4.43 
0.29 

80 

77 
93 
92 

20 

23 

7 
8 

10.25 

25 

muffle 
increased 

4.68 

10  29 

25 

1  36 

10  27 

25 

increased 

10  42 

10.17 

25 

back  of  muffle 

16.43 

TABLE  XXXIII.— CUPELLATION  OF  GOLD 

(ROSE2) 


Temp,  of  cup- 

Amt.  of  gold 

Amt.  of  silver 

Amt.  of  Pb 

ellation  deg. 

Total  loss  gold 

mgs. 

mgs. 

grams 

Cent. 

per  cent. 

air  in  muffle 

1 

4 

25 

900° 

1.2 

1 

6 

25 

900 

1.05 

1 

8 

10 

900 

0.90 

1 

10 

25 

900 

0.80 

1 

6 

25 

700 

0.45 

1 

10 

25 

700 

0.39 

500 

1250 

10 

900 

0.055 

1  Bull.  No.  253,  U.  S.  G.  Survey. 
*Eng.  and  Min.  Jour.,  LXXIX,  708. 


166 


A    MANUAL    OF   FIRE    ASSAYING 


TABLE  XXXIV.— CUPELLATION  OF  GOLD-SILVER  ALLOTS 

(HlLLEBRAND    AND    ALLEN) 
ALL    CUPELLATIONS    MADE    WITH    25    GRAMS    OF   LEAD 


Amount 
gold 
mgs. 

Amount 
silver 
mgs. 

Temp.  6C. 

Total  loss 

Ab1uSby!v°"" 

Gold 
per 
cent. 

SUver 
per 
cent. 

Gold 
per 
cent. 

Silver 
per 
cent. 

Gold 
per 
cent. 

Silver 
per 
cent. 

30.06 
30.40 
30.60 
30.07 
30.61 
15.56 
15.14 
15.15 
15.44 
15.52 
15.39 
10.67 
10.57 
10.53 
10.63 
10.60 
10.21 

90.51 
90.19 
90.74 
90.67 
90.75 
45.06 
45.19 
45.41 
•45.30 
45.59 
45.05 
30.33 
30.64 
30.42 
30.52 
30.38 
30.44 

750°  (air  in  muffle) 

0.50 
1.22 
2.32 
3.76 
3.89 
0.19 
0.40 
1.52 
2.07 
2.59 
2.40 
0.47 
1.61 
5.13 
10.63 
12.46 
12.53 

1.70 
3.73 
5.51 
7.66 
7.98 
1.91 
3.30 
4.14 
5.78 
6.55 
6.61 
2.17 
5.68 
10.19 
15.99 
18.34 
18.69 

33 
17 
48 
33 
33 
27 

19 
22 
37 
38 
30 
29 

increased  
back  of  muffle.  .  .  . 
front,  750°  
increased  
increased  
increased  
increased  
back  of  muffle.  .  .  . 
front,  750°  

67 
83 
52 
67 
67 
73 

81 
78 
63 
62 
70 
71 

increased  
increased  
increased  
back  of  muffle.  .  .  . 





Rose  shows  (Table  XXXIII)  the  protective  action  that  silver 
exercises  over  gold,  the  total  loss  of  gold  decreasing  as  the  amount 
of  silver  present  increases.  Hillebrand  and  Allen  show  how  the 
total  loss  is  distributed  between  absorption  by  the  cupel  and 
volatilization.  It  is  evident  that  while  the  total  loss  of  gold  is 
decreased  by  the  presence  of  silver,  the  volatilization  loss  of  gold 
is  increased  by  the  presence  of  silver  (compare  Tables  XXXII  and 
XXXIV) .  When  gold  and  silver  are  present  in  the  ratio  of  1  to 
2,  the  averages  are  as  follows: 

Of  the  total  gold  loss,  68  per  cent,  is  absorbed,  32  per  cent,  is 
volatilized. 

Of  the  total  silver  loss,  71  per  cent,  is  absorbed,  29  per  cent, 
is  volatilized. 

However,  as  the  total  loss  is  determined  by  the  difference  in 
weight  between  the  proof  gold  and  silver  and  the  weights  of  the 
cupelled  bead  and  parted  gold,  and  the  volatilization  item  by 
the  difference  between  the  total  loss  and  the  amount  recovered 
by  the  re-assay  of  the  cupel,  it  is  evident  that  certain  errors  obtain 
which  apparently  make  the  volatilization  loss  appear  greater 


ERRORS    IN    THE   ASSAY    FOR    GOLD   AND    SILVER 


167 


than  it  really  vis.  The  error,  however,  cannot  be  very  great. 
The  data  are  inconclusive  regarding  the  influence  of  the  tempera- 
ture on  the  relative  losses  by  absorption  and  volatilization,  but 
it  seems  indicated  that  the  volatilization  loss  is  proportionately 
greater  with  an  increase  of  the  temperature  sof  cupellation. 

LOSSES  IN  THE  ASSAY  OF  ORES.— Table  XXXV,  etc.,  show 
losses  of  gold  and  silver  in  the  assay  of  ores,  during  fusion  and 
cupellation,  as  influenced  by  the  presence  of  certain  impurities. 

TABLE  XXXV.— TELLURIDE  ORES 


Amount  of  gold  in  weight 
of  ore  taken  for  assay 

Weight  of 
lead  button 

Method  of 
fusion. 

Slag  loss 

Cupel 
absorption 

Milligrams 

Grams 

Per  cent. 

Per  cent. 

(l)493.83 

20 

Crucible7 

0.51 

1.56 

95.57 

20 

Crucible 

0.38 

0.23(5) 

1.54 

20 

Crucible 

1.30 

0.40 

1.95 

20 

Crucible 

0.50 

0.50 

1.19 

20 

Crucible 

0.40 

1.17 

20 

Crucible 

0.40 

3.60 

20 

Crucible 

2.14 

0.80 

6.20 

20 

Crucible 

0.64 

0.32 

6.23 

20 

Crucible 

0.50 

0.64 

1.38 

20 

Crucible 

0.80 

1.00 

(2)   18.18 

25 

Crucible 

0.49 



5.85 

25 

Crucible 

1.03 

0.12(«) 

(3)  34.0 

27 

Crucible 

0.21 

0.23 

34.0 

27 

Crucible 

0.56 

34.0 

25 

Crucible 

0.15 

0.41 

68.0 

25 

Crucible 

0.13 

0.07 

68.0 

25 

Crucible 

0.16 

0.22 

(*)   15.5 

Crucible8 

0.25 

0.19 

15.49 

Crucible 

0.13 

0.38 

19.54 

Crucible 

0.20 

0.23 

19.63 

' 

Crucible 

0.10 

0.25 

Woodward,  in  "West.  Chem.  and  Mel.,"  I,  12. 

Fulton,  in  "School  of  Mines  Quart.,"  XIX,  419. 

Lodge,  in  "Tech.  Quart."  1899,  XII,  171  (averages). 

Bull.  No.  253,  U.  S.  G.  Survey  (averages;  Hillebrand  and  Allen). 

Average  of  34  fusions,  tellurium  in  all  beads. 

Average  of  10  fusions. 

Cripple  Creek  flux. 

Excess-litharge  charge. 


168 


A    MANUAL    OF   FIRE    ASSAYING 


TABLE  XXXVI.— ZINCIFEROUS  MATERIAL,  ETC. 


Amount  of  Au 

and    Ag    in 
weight  of  ore 
taken  for  as- 

Weight 
of  lead 
button 

Slag  loss 

Cupel 
absorption 

say. 

Method  of  assay 

Remarks 

Au 

Ag 

Au 

Ag 

Au 

Ag 

mgs. 

mgs. 

Grams 

per 

cent. 

per 
cent. 

per 
cent. 

per 

cent. 

"232.0 

287.0 

18 

Scorification    after 

0.06 

0.40 

0.11 

1.30 

Zn.  ppt.  containing 

acid  treatment 

42.3  per  cent.  Zn 

232.0 

284.0 

18 

Scorification    after 

0.04 

0.34 

0.08 

1.10 

Figures  represent 

acid  treatment 

averages. 

232.0 

284.0 

18 

Direct  crucible 

1.04 

1.10 

0.16 

1.50 

fusion 

2233.0 

197.0 

20 

Crucible  fusion 

0.06 

0.51 

0.18 

1.18 

Zn.  ppt.  containing 

after    acid    treat- 

14.3   per  cent.  Zn, 

ment 

9.1  per  cent.  Cu. 

233.0 

202.0 

20 

Direct  crucible 

0.15 

2.73 

0.16 

1.29 

Figures  represent 

fusion 

averages 

3 

561.0 

20 

Crucible  fusion 

0.75 

1.38 

Galena 

niter  method 

567.0 

20 

Crucible  fusion 

0.65 

1.42 

Galena 

niter  method 

3 

175.0 

20 

Crucible  fusion 

0.23 

1.90 

Silicious     ore     con- 

niter method 

taining  some  copper 

174.0 

20 

0.37 

1.66 

Silicious     ore     con- 

taining some 

copper 

1  Fulton  and  Crawford,  in  School  of  Mines  Quart.,  XXII,  153 

2  Lodge,  in  Trans.  A.  I.  M.  E.,  XXXIV,  432. 
»  Miller,  in  School  of  Mines  Quart,  XIX,  43 


ERRORS    IN    THE    ASSAY    FOR    GOLD    AND    SILVER 


169 


TABLE  XXX  VII  (>).— HIGH-GRADE  CARBONATE  AND  SULPHIDE 
SILVER   ORES2 


Amt.    of  Au 

and  Ag  in 

weight  of  ore 
taken  for 

Weight 
of  lead 
button 

Method 

Slag  loss 

Cupel 
absorption 

Second      correction 

assay 

of 

from  fusion  of  slags 

assay 

anu  cupels   01  nrst 

Au 

Ag 

Grams 

Au 
per 

Ag 
per 

Au 
per 

Ag 
per 

correction  per  cent. 

mgs. 

cent. 

cent 

cent. 

cent. 

1130 

24 

Crucible  fusion 

0.76 

1.25 

0.248 

1130 

28 

Crucible  fusion 

0.361 

0.956 

Crucible  fusion 

• 

1130 

28 

Double   amt.    of 

0.511 

0.88 

0.15 

fluxes 

Crucible  fusion 

1130 

32 

•   Double   amt.    of 

0.736 

1.02 

0.15 

fluxes 

226 

15 

Scorification 

2.8 

1.40 

0.559 

24 

1719 

25 

Crucible  fusion 

0.05 

0.143 

0.07 

0.711 

12 

860 

20 

Scorification 

0.12 

0.72 

0.080.86 

TABLE  XXXVIII.— CUPRIFEROUS  MATERIAL 


Amt.    of   Au 

and  Agin 
weight  of  ore 
taken  for 

Weight 
of  lead 
button 

Method 

Total  loss  recovered 
including 
slag  and  cupel 

assay 

of 
assay 

Remarks 

Au 

Ag 

Grams 

Au 

Ag 

mgs. 

mgs. 

per  cent. 

per  cent. 

i 

11.46 

88.0 

20 

Crucible  fusion 

0.96 

2.90 

Mattes  containing 

about  20  per  cent.  Cu. 

5.36 

46.8 

20 

Crucible  fusion 

2.98 

4.30 

4.20 

37.24 

20 

Crucible  fusion 

2.38 

2.70 

3.52 

33.79 

20 

Crucible  fusion 

2.84 

5.47 

4.20 

37.24 

20 

Scorification 

5.95 

5.26 

3.52 

33.79 

20 

Scorification 

3.12 

5.71 

The  foregoing  tables  represent  for  the  most  part  averages, 
and  in  every  case  the  losses  for  the  normal  assay;  i.e.,  in  the  case 
of  the  fusion,  the  charge  known  to  yield  the  best  results,  and 
the  proper  temperature  for  cupellation.  The  losses  are  therefore 

1  First  five  results  on  lead  carbonate  ore,  last  two  on  silver  sulphides.  All  results  repre- 
sent averages. 

1  Miller  and  Fulton,  ibid,  XVII,  160. 


170  A   MANUAL    OF    FIRE    ASSAYING 

to  be  ascribed  to  the  nature  of  the  material  assayed,  chiefly  to 
the  influence  of  certain  elements  present.  In  considering  the 
percentage  of  loss,  it  must  be  recalled  that  this  varies  inversely 
with  the  amount  of  precious  metal  in  the  charge,  i.e.,  with  the 
size  of  the  gold-silver  bead.  The  sum  of  the  cupel  absorption 
and  the  slag  loss  (which  can,  in  part,  be  recovered)  is  not  the 
total  loss,  as  it  does  not  include  that  by  volatilization,  which  is 
small  in  most  cases,  but  in  some  cases,  again,  may  be  quite  appre- 
ciable, as  in  the  case  of  telluride  ores.  What  the  loss  is  in  slag, 
when  no  element  like  tellurium,  copper,  zinc,  etc.,  is  present,  may 
be  seen  by  reference  to  Table  XXXVI,  to  those  assays  fused  after 
acid  treatment,  and  to  TableXXXVII,  showing  crucible  fusions  on 
lead  carbonate  ore.  The  slag  loss  in  gold  and  silver  for  these  ores 
is  very  small.  In  cases  where  the  impurity  present  and  causing 
loss  is  nearly  all  eliminated  in  the  fusion,  e.g.,  zinc,  antimony, 
etc.,  the  cupel  absorption  is  practically  that  for  pure  silver  and 
gold  under  the  same  circumstances.  Where  the  impurity  is 
tellurium,  or  selenium,  or  copper,  the  cupel  absorption  is  decidedly 
increased.  One  fact  is  to  be  noted,  the  fact  that  the  slag  losses 
present  no  regularity,  even  for  the  same  material.  This  is  prob- 
ably due  partly  to  differences  of  slag  composition  among 
different  experimenters,  and  partly  to  difference  of  temperature 
of  fusion;  and  also  to  the  method  of  refusion  of  slag. 

The  high  loss  in  scorification  slags  shown  in  Table  XXXVII  for 
lead  carbonate  ores  containing  silver  is  due  to  the  general  un- 
suitability  of  the  ore  for  scorification,  although  scorification 
slags  show  higher  losses  than  crucible  slags.  That,  in  spite  of 
this,  scorification  assays,  on  silver-bearing  material  show  equally 
good  and  better  results  in  many  cases  than  the  crucible  assay,  is 
due  to  the  fact  that  the  silver  beads  retain  small  quantities  of 
lead  and  copper  (see  further  on),  and  to  the  fact  that  in  the  multi- 
plication of  the  weight  of  the  silver  bead  by  5  or  10,  or  whatever 
the  assay-ton  factor  may  be,  this  error  is  multiplied,  giving  an 
apparently  better  result. 

The  amount  of  slag  has  comparatively  little  influence  on  the 
amount  of  precious  metals  retained,  provided  the  amount  of 
collecting  lead  is  ample.  Buttons  of  less  than  18  to  20  grams 
should  not  be  made,  and  if  the  amount  of  slag  is  great  or  the 
quantity  of  silver  and  gold  in  the  qharge  is  more  than  500  mgs., 
25-  and  30-gram  buttons  are  essential.  In  the  case  of  large 
buttons  which  contain  no  impurity,  it  is  also  best  to  cupel  direct, 


ERRORS    IN    THE   ASSAY   FOR    GOLD   AND    SILVER  171 

if  possible,  rather  than  rescorify  to  smaller  size,  as  the  rescorifi- 
cation  causes  greater  loss  than  the  direct  cupellations. 

During  scorification  there  is  also  an  appreciable  loss  of  the 
precious  metals  by  volatilization,  which  is  absent  in  the  crucible 
assay.  This,  in  the  case  of  telluride  or  zinciferous  ores,  may 
become  so  great  as  to  put  scorification  out  of  the  question. 

OTHER  ERRORS. — Retention  of  Lead  in  Cupelled  Beads. — Small 
quantities  of  lead  are  almost  invariably  retained  in  the  gold  and 
silver  beads  with  ordinary  temperatures  of  cupellation.  Hille- 
brand  and  Allen,1  in  two  careful  experiments  on  sets  of  three 
beads,  approximately  together  90  mgs.  gold,  found  that  0.30  per 
cent,  and  0.37  per  cent.,  respectively,  of  lead  were  retained. 
This  retention  of  lead  cannot  be  corrected  by  leaving  the  bead 
in  the  muffle  for  some  length  of  time  after  the  blick,  as  this  is,  of 
course,  prohibitive  in  the  case  of  silver,  and  in  the  case  of  gold 
seems  to  actually  cause  an  increase  of  weight.  It  has  already 
been  stated  that  copper  and  tellurium  are  very  apt  to  be  present 
in  the  final  bead,  when  in  the  ore  in  any  appreciable  quantity. 
The  retention  of  base  metal  by  the  bead  causes  a  plus  error  in 
silver,  but  will  not  effect  the  result  on  gold  unless  the  parting  is 
by  H2SO4;  and  where  the  weight  of  the  bead  is  multiplied  by  a 
factor  to  get  results  per  ton,  the  final  error  in  silver  may  be  very 
appreciable.  The  presence  of  copper  in  the  final  bead  practically 
insures  the  complete  removal  of  the  lead. 

In  order  to  show  what  is  usually  termed  "fine  silver"  the  follow- 
ing analysis  of  Government  fine  silver  is  appended. 

Ag,  99.929%;  Cu,  0.056%;  Pb,  0.003%;  Au,  0.007%;  As, 
0.001%;  Sb,  0.002%;  Fe,  0.001%;  Zn,  trace.2 

Retention  of  Silver  by  the  Parted  Gold. — Ordinary  parted  gold, 
after  the  proper  treatment  with  weak  and  strong  acid,  retains 
from  0.05  to  0.10  per  cent,  of  silver.  In  the  assay  of  gold  bullion 
after  the  first  acid  treatment  of  the  quartation  alloy,  the  gold 
on  the  average  retains  0.25  per  cent,  silver.  After  the  second 
acid  treatment,  the  final  silver  retention  is  from  0.06  to  0.09 
per  cent.,  depending  on  the  time  of  boiling.  If  the  amount  of 
silver  to  gold  in  the  quartation  alloy  is  less  than  2.5  to  1,  some- 
what more  than  the  above  amount  of  silver  will  be  retained.3 

Silver  can,  practically,  be  completely  extracted  by  more  than 

»  Bull.  No.  253,  U.  S.  G.  Survey. 

*  Min.  Ind.,  XV,  545. 

3  Rose,  "Metallurgy  of  Gold,"  p.  453. 


172  A   MANUAL    OF    FIRE    ASSAYING 

two  treatments  with  acids,  according  to  Hillebrand  and  Allen.1 
In  the  ordinary  assay  for  ores  as  usually  carried  out,  it  is  safe  to 
assume  that  some  silver  is  invariably  retained  by  the  gold,  and 
frequently  much  more  than  is  supposed;  however,  with  low- 
grade  ores,  this  retention  is  negligible. 

Solution  of  Gold  by  Acid. — It  is  essential  that  the  nitric  acid 
used  for  parting  be  free  from  impurities,  especially  from  hydro- 
chloric acid  and  chlorine;  otherwise  solution  of  gold  is  sure  to 
follow.  Gold  is  quite  soluble  in  mixtures  of  hot  sulphuric  and 
nitric  acid,2  and  is  again  precipitated  by  dilution. 

According  to  Hillebrand  and  Allen,3  nitrous  acid  (HNO2)  and 
mixtures  of  HNO3  and  HNO2  do  not  dissolve  gold,  though  there 
is  much  earlier  literature  to  the  contrary.  Nitrous  acid  has 
frequently  been  considered  in  this  connection,  as  it  is  formed  to 
some  extent  by  the  action  of  HNO3  on  silver. 

According  to  Rose,4  some  gold  is  dissolved  by  nitric  acid  on 
continued  boiling  to  constant  gravity  of  acid.  This  solution  is 
placed  in  the  bullion  assay  at  0.05  per  cent,  or  0.5  parts  per  1000. 
Hillebrand  and  Allen  state  that  the  loss  of  gold  by  solution  is 
very  small  and  irregular.  It  may  be  disregarded  in  the  ore  assay. 
The  solubility  of  gold  in  HNO3  is  readily  demonstrated  when  large 
quantities  of  gold  are  used.  F.  P.  Dewey5  in  careful  experiments 
showed  the  solution  of  gold  to  the  extent  of  660  mgs.  per  liter  of 
of  cone,  acid,  on  boiling  about  25  grams  gold  .for  two  hours.  He 
states  that  the  temperature  (120°  C.)  required  to  boil  cone,  acid 
has  as  decided  an  influence  as  the  strength  of  the  acid. 

Occluded  Gases. — Parted  gold  beads  and  "cornets"  retain 
about  twice  their  volume  in  occluded  gases  after  annealing.  The 
principal  gas  is  stated  to  be  carbon  monoxide.  Two  volumes 
amount  to  0.02  per  cent,  by  weight,  which  is  already  allowed  for 
in  the  silver  retention. 

Errors  in  Weighing. — The  best  scales  are  accurate  to  0.01  mg., 
and  scales  can  be  obtained  weighing  to  0.005  mg.  This  last  is 
used  in  assay  offices,  where  great  accuracy  is  required,  on 
such  material  as  bullions,  rich  mattes,  etc.  It  is  usually  an 
unnecessary  refinement  in  the  ordinary  ore  assay,  for  the  reason 
that  the  probable  error  in  the  assay  is  greater  than  this. 

1  Bull.  253,  U.  S.  G.  Survey. 

2  Lenher,  in  "Jour.  Am.  Chem,  Soc.,"  XXVI,  552 

3  Ibid. 

4  Ibid.,  p.  507. 

6  Jour.  Am.  Chem.  Soc.,  XXXII,  318. 


ERRORS    IN   THE    ASSAY    FOR   GOLD    AND    SILVER  173 

The  errors  in  the  assay  for  gold  and  silver  may  be  summarized 
as  follows: 

1.  Losses  by  absorption  in  the  slag  of  the  fusion. 

2.  Losses  by  volatilization  during  fusion. 

3.  Losses  by  absorption  during  cupellation. 

4.  Losses  by  volatilization  during  cupellation. 

5.  Errors  by  gain  in  weight  of  bead,  due  to  retention  of  foreign 
elements.     This  affects  results  on  silver  chiefly. 

6.  Errors  in  weight  of  gold  after  parting  by  the  retention  of 
silver  and  occluded  gases. 

7.  Losses  of  gold  by  solution  in  nitric  acid. 

8.  Errors  in  weighing. 

The  chief  losses  are  Nos.  1  and  3,  which  can  be  recovered  by 
"corrected  assay,"  i.e.,  by  re-assay  of  slag  and  cupel,  to  the  ex- 
tent of  about  80  to  85  per  cent.  Wherever  considerable  accuracy 
is  required,  corrected  assays  should  always  be  made.  The  losses 
by  volatilization  are  usually  slight,  although  from  the  foregoing 
data  these  are  sometimes  seen  to  be  considerable.  The  retention 
of  foreign  metals  by  the  bead  is  a  plus  error  in  favor  of  silver,  and 
the  retention  of  silver  in  the  parted  gold  is  a  plus  error  in  favor  of 
gold.  Silver  losses  are  considerably  greater  in  magnitude  than  gold 
losses.  The  total  amount  of  precious  metal  recovered  by  the 
assay  varies  with  the  nature  of  the  material.  Designating  the 
total  amount  of  gold  and  silver  in  an  ore  or  produet  as  100, 
the  corrected  assay  will  show  from  99  to  99.8  per  cent,  of  the 
gold,  and  from  98  to  100+  per  cent,  of  the  silver,  the  high  silver 
result  in  some  cases  being  due  to  retention  of  foreign  metal. 

In  the  bullion  assay  for  gold,  the  algebraic  sum  of  the  errors 
outlined,  the  losses  being  designated  minus  and  the  gains  plus, 
is  called  the  "surcharge."  In  the  gold  bullion  assay  this  will 
vary  from  +0.025  per  cent,  in  very  pure  gold  bullion,  to  —0.25 
per  cent,  in  base  bullion,  passing  to  zero  for  a  bullion  about 
800  fine. 


CHAPTER  XII 
THE  ASSAY  OF  BULLION 

GENERAL. — Bullion  is  classified  as  follows: 

1.  Lead  bullion,  usually  the  product  of  the  lead  blast-furnace; 
95  per  cent,  and  more  lead,  containing  some  copper,  antimony, 
etc.,  silver  and  gold. 

2.  Base  bullion,  containing  from  100  to  925  parts  of  silver 
per  1000,  gold  in  varying  amounts,  and  a  large  percentage  of 
base  metals,   chiefly  copper,   zinc,   lead,   etc.     Produced  most 
frequently  by  cyanide  mills. 

3.  Dore   bullion,   containing   925  to  990  parts  of  silver  per 
1000,  some  gold,  and  base  metals,  mostly  copper,  but  also  lead, 
antimony,  zinc,  etc. 

4.  Fine  silver  bullion,  free  from  gold,  containing  990  and  more 
parts  silver  per  1000,  but  some  base  metals,  usually  copper. 

5.  Silver  bullion,  containing  little  base  metal  and  less  than 
half  its  weight  in  gold. 

6.  Gold  bullion,  containing  little  base  metal  and  more  than 
half  its  weight  in  gold. 

7.  Fine  gold  bullion,  free  from  silver,  containing  from  990  to 
1000  parts  gold  per  1000. 

Silver  and  gold  in  all  bullions  but  lead  bullion  are  estimated 
in  parts  per  thousand,  and  bullion  is  said  to  be  so  many  parts 
fine.  Thus,  if  1  gram  (1000  mgs.)  of  bullion  is  taken  for  assay 
and  it  contains  925  mgs.  gold,  it  is  said  to  be  925  fine. 

In  the  assay  of  gold  bullion  the  "millieme"  system  of  assay 
weights  is  used,  a  millieme  being  0.  5  mg.,  and  the  assay  is  re- 
ported in  parts  of  10,000,  or  the  fineness  with  one  decimal  added. 
Thus  the  above  bullion  would  be  reported  as  925.0  fine.  In 
this  system  the  500-mg.  weight  is  stamped  1000,  the  250-mg. 
weight  500,  etc.  The  scales  used  must  therefore  be  sensitive 
to  0.05  mg.,  or  0.1  millieme.  This  presents  no  difficulty,  as 
ordinary  assay  balances  are  sensitive  to  0.01  mg.  with  a  load  of 
0.5  gram. 

Lead  bullion  is  recorded  in  oz.  per  ton,  in  the  same  way  as 
for  ores. 

174 


THE    ASSAY    OF    BULLION  175 

THE  ASSAY  OF  LEAD  BULLION.— The  sample  of  bullion  may 
be  melted  under  charcoal  and  granulated  in  cold  water,  or  it  may 
be  rolled  out  into  a  strip  in  the  rolls,  and  the  pieces  cut  at  inter- 
vals from  this  for  the  sample.  If  lead  bullion  is  free  from  copper, 
antimony,  zinc,  sulphur  and  arsenic,  etc.,  it  may  be  cupelled 
directly  for  gold  and  silver.  In  this  case,  4  portions  of  0.5  assay 
ton  each  are  wrapped  in  about  7  grams  of  sheet  lead,  placed  in 
the  hot  cupels,  and  cupelled  with  feathers.  The  cupels  are  fused 
with  the  following  charge: 

Stained  part  of  cupel  45  grams  borax  glass 

80  grams  PbO  2  grams  argol 

15  grams  Na2CO3  Thin  litharge  cover 

The  buttons  from  this  fusion  are  cupelled  and  the  weight  of 
the  gold  and  silver  added  to  that  obtained  from  the  first  cupella- 
tion. 

If  the  bullion  contains  base  metals  which  will  influence  the 
results  of  the  cupellation,  4  portions  of  either  0.5  or  1.0  assay 
ton  are  weighed  out  and  mixed  with  30  to  50  grams  of  test  lead; 
1.5  grams  of  borax  glass  and  0.5  gram  of  silica  are  put  on  top  of 
the  lead  and  the  charge  scorified.  The  resultant  buttons,  which 
should  weigh  about  15  grams,  are  then  cupelled.  The  scorifier 
slag  and  cupel  are  re-assayed  by  the  above  charge  and  the  correc- 
tion added. 

THE  ASSAY  OF  SILVER  BULLION1  (also  applicable  to  Base  Bul- 
lion, Dore  Bullion,  etc.).  CUPELLATION  METHOD.— This  method 
is  used  as  an  approximation  for  bullions  in  which  silver  is  to  be 
determined  accurately,  serving  as  a  preliminary  assay  for  the 
salt  titration,  mint,  or  Gay-Lussac  method. 

(a)  Preliminary  Assay. — Exactly  500  mgs.  of  bullion  are 
weighed  out  on  an  assay  balance  in  order  to  save  calculation, 
wrapped  in  10  grams  of  sheet  lead,  and  cupelled  at  850°.  C.,  or 
with  ample  feathers  of  litharge.  The  silver  bead  is  cleaned, 
weighed  and  parted  in  1  to  9  HNO3  for  at  least  20  minutes;  then, 
if  any  gold  shows,  heated  for  5  minutes  more  in  concentrated 
acid,  washed,  and  the  gold  dried,  annealed  and  weighed.  The 
amount  of  gold  found,  subtracted  from  the  weight  of  the  bead, 
gives  the  approximate  silver,  and  the  weight  of  the  bead,  sub- 
tracted from  the  amount  of  bullion  taken  (500  mgs.),  gives  the 

1  For  sampling  of  silver  bullion,  see  "The  Assay  of  Gold  Bullion,"  later  in  this  Chapter. 


176 


A    MAXUAL    OF    FIRE    ASSAYING 


base  metal.     This  base  metal  is  usually  copper,  and  its  presence 
may  be  detected  by  the  coloring  of  the  cupel. 

(&)  Making  the  Check  Assay. — As  the  loss  of  silver  and  gold 
is  a  question  of  temperature,  amount  of  precious  metal  present, 
amount  of  lead  of  cupellation,  and  amount  and  kind  of  base 
metal  present,  it  is  desirable  to  have  the  regular  cupellation, 
accompanied  by  a  check  assay,  made  up  as  nearly  as  possible  to 
the  composition  of  the  bullion  to  be  assayed,  and  cupelled  under 
the  same  conditions.  The  check  assay  is  therefore  made  up  from 
data  obtained  in  the  preliminary  assay.  As  the  silver  determined 
in  this  preliminary  assay  is  low,  due  to  absorption  and  volatiliza- 
tion, a  correction  of  1.2  per  cent,  is  added  as  an  approximation 
or,  rather,  the  amount  of  Ag  found  is  considered  as  98.8  per 
cent,  of  that  present,  and  this  amount  of  proof  silver  weighed 
out.  To  this  is  added,  in  proof  gold,  the  amount  of  gold  found 
in  the  preliminary  assay.  The  difference  between  the  sum  of 
the  corrected  silver  and  the  gold,  and  500,  is  the  amount  of  base 
metal  to  be  weighed  out  for  the  check.  As  already  stated,  the 
base  metal  is  usually  copper,  and  in  making  up  the  check  c.p. 
sheet  copper  is  used.  The  check  thus  weighs  500  mgs.  and  approx- 
imates very  closely  the  composition  of  the  bullion.  Duplicates 
of  500  mgs.  of  bullion  are  now  weighed  out,  and  these  and  the 
check  each  wrapped  in  the  proper  amount  of  sheet  lead,  as  de- 
termined from  the  table  below: 

TABLE  XXXIX.— LEAD  RATIO  IN  CUPELLATION 


Fineness  in 

Amount  of 

Amount  of  lead 

silver 

copper  present 

for  cupellation 

Ratio  of  lead 

Milliemes 

Milliemes 

Grams 

1000 

0 

3 

900 

100 

7 

140  to  1 

800 

200 

12 

120  to  1 

500 

500 

18 

72  to  1 

300 

700 

.21 

60to  1 

(c)   The  Assay. — Three  cupels  are  placed  in  a  row  across  the 
muffle,  so  as  to  be  exposed  as  nearly  as  possible  to  the  same 


THE    ASSAY    OF    BULLION  177 

temperature,  and  three  more  cupels  are  placed  near  them  to  act 
as  covers  for  the  cupellation  when  finished,  in  order  to  prevent 
sprouting.  When  the  cupels  have  had  all  volatile  matter  expelled 
the  assays  are  dropped  into  them,  the  check  in  the  center  one, 
and  the  cupellations  carried  on  in  the  usual  way,  with  feathers. 
After  the  blick,  the  cupels  are  drawn  to  the  front  of  the  muffle 
and  covered  with  extra  cupels.  Sprouted  buttons  must  be  re- 
jected. The  beads  are  now  cleaned,  weighed,  and  rolled  out, 
parted  in  flasks,  with  the  acids  as  described  for  the  preliminary 
assay,  and  the  gold  weighed. 

The  difference  between  the  silver  actually  used  in  the  check 
and  that  found  by  assay  is  the  correction  to  be  added  to  the 
mean  silver  result  of  the  two  bullion  assays  made,  which  should 
not  differ  by  more  than  a  millieme  (0.5  point  fineness).  This 
correction  may  be  plus  or  minus,  according  to  the  amount  of 
copper  in  the  bullion;  for  with  much  copper,  some  of  this  may 
be  retained  by  the  silver  and  give  rise  to  a  minus  correction. 
The  gold  is  corrected  in  the  same  way  as  the  silver.  The  sub- 
traction from  500  of  the  sum  of  the  corrected  silver  and  gold 
gives  the  amount  of  base  metal.  The  individual  results  obtained, 
express  the  assay  results  in  fineness. 

When  metals  of  the  platinum  group  are  present,  the  method 
"must  be  modified  as  outlined,  in  Chapter  XIII,  for  the  assay  of 
platinum,  etc. 

WET  METHODS:  GAY-LUSSAC  OR  MINT  METHOD. — This 
method  is  a.  most  accurate  one  and  is  based  on  the  complete 
precipitation  of  Ag  as  AgCl  in  a  nitric  acid  solution  by  means  of 
sodium  chloride.  The  reaction  is  as  follows: 

AgN03  +  NaCl  =  AgCl  +  NaN03 
1  part  Ag  =  0.54207  NaCl 

The  standard  solution  of  NaCl  usually  employed  is  of  such 
strength  that  100  c.c.  precipitate  1  gram  of  Ag,  so  that  5.4207 
grams  of  c.p.  NaCl  are  dissolved  per  liter  of  distilled  water  to 
give  the  standard  solution.  This  solution  can  also  be  made  up 
by  using  a  saturated  salt  solution  at  60°  F.,  and  then  adding 
2.07  parts  of  this  to  97.93  parts  of  distilled  water.  The  last 
method  of  obtaining  the  solution  is  not  as  good  as  the  first,  owing 
to  the  difficulty  of  obtaining  the  precise  temperature  of  60°  F. 
and  keeping  it  there.  Aside  from  the  standard  solution  men- 
tioned, there  is  required  another  of  one-tenth  its  strength  (obtained 


178  A   MANUAL    OF    FIRE   ASSAYING 

by  taking  1  part  of  the  standard  NaCl  solution  and  adding  to  it 
9  parts  of  distilled  water),  and  an  acidulated  solution  of  AgN03, 
obtained  by  dissolving  1  gram  of  proof  silver  in  15  c.c.  of  HN03, 
1.26  sp.  gr.,  and  diluting  with  distilled  water  to  1000  c.c.  It 
follows  from  the  above  that  1  c.c.  of  the  one-tenth  solution  will 
just  precipitate  the  Ag  in  1  c.c.  of  the  acidulated  silver  nitrate 
solution. 

The  standard  NaCl  solution  is  termed  the  "normal  salt" 
solution  in  the  assay,  although  not  properly  so;  the  weak  solution 
is  termed  the  "decimal  salt  solution,"  and  the  silver  nitrate 
solution  the  "decimal  silver"  solution. 

Standardizing  Solutions. — The  apparatus  required  is: 

1.  A  large  bottle  or  carboy,  containing  the  normal  salt  solution 
placed  on  an  elevated  shelf  so  that  the  solution  may  be  si- 
phoned by  means  of  glass  tubing  and  rubber  hose  to  the  main 
100-c.c.  pipette. 

2.  Liter  bottles  containing  respectively  the  decimal  salt  and 
the  decimal  silver  solutions. 

3.  An  accurate  100-c.c.  pipette,  clamped  to  a  suitable  stand, 
and  provided  at  the  top  with  a  glass  overflow-cup  containing  a 
moistened    sponge   to  catch    the  overflow  of    the  normal  salt 
solution. 

4.  Two  small  graduated  10-c.c.  pipettes,  one  for  the  decimal 
salt  and  one  for  the  decimal  silver  solution.     Burettes  may  be 
used  in  place  of  these. 

5.  A  number  of  strong  8-  to  12-oz.  bottles,  similar  to  reagent 
bottles,  provided  with  rubber  corks. 

The  standardizing  of  solutions  is  carried  out  as  follows:  Two 
portions  of  exactly  1002  mgs.  proof  silver  are  dissolved  in  15  c.c. 
of  1.26  sp.  gr.  HNO3,  the  nitrous  fumes  are  removed  by  boiling, 
the  solution  is  transferred  to  the  titration  bottles  and  water 
added  to  bring  up  the  amount  of  solution  to  125  c.c.  The  100-c.c. 
pipette  is  then  rilled  with  normal  salt  solution  to  the  mark,  after 
washing  out  with  salt  solution  to  prevent  dilution.  The  filling 
is  done  by  fastening  the  siphon  hose  to  the  bottom  of  the  pipette, 
opening  the  clamp  on  the  hose,  and  letting  the  pipette  fill,  with  a 
little  overflow.  The  solution  is  then  shut  off  by  clamping  the 
hose,  a  finger  placed  on  the  top  opening  of  the  pipette  to  prevent 
the  solution  running  out,  and  the  hose  removed.  The  pipette 
is  then  permitted  to  drain  to  the  100-c.c.  mark,  and  the  solution 
held  there  by  closing  the  top  of  the  pipette  with  the  finger.  The 


THE   ASSAY    OF    BULLION  179 

bottle  containing  the  dissolved  proof  silver  is  then  placed  under 
the  pipette  and  the  normal  salt  solution  permitted  to  completely 
drain  into  it.  The  bottle  is  then  violently  shaken  for  three  or 
four  minutes,  either  by  hand  or  a  mechanical  agitator,  and  the 
AgCl  allowed  to  settle,  leaving  the  supernatant  liquid  clear.  If 
the  normal  solution  is  made  up  correctly,  it  will  have  precipitated 
just  1000  mgs.  of  silver,  leaving  2  mgs.  unprecipitated.  One  c.c. 
of  decimal  salt  solution  is  now  added  to  the  bottle  by  means  of 
one  of  the  10-c.c.  pipettes  or  a  burette,  which,  if  the  solution 
still  contains  Ag  unprecipitated,  gives  rise  to  a  white  cloud  of 
AgCl.  The  bottle  is  again  shaken,  the  precipitate  allowed  to 
settle,  and  another  c.c.  of  decimal  salt  solution  added.  If  this 
fails  to  give  a  precipitate,  then  100.1  c.c.  of  normal  salt  solution 
are  equivalent  to  1002  mgs.  of  silver  (1  c.c.  of  decimal  salt  solution 
=  0.1  c.c.  normal  salt  solution).  If  the  second  addition  of 
decimal  salt  solution  gives  a  precipitate,  the  shaking  and  settling 
are  repeated,  and  a  third  and  fourth,  etc.,  addition  made,  until  no 
further  cloud  appears.  The  assayer  soon  learns  to  judge  by  the 
density  of  the  cloud  whether  only  part  of  the  c.c.  has  been  used 
up.  In  this  way  he  should  be  able  to  judge  to  the  fourth  of  a 
c.c.  or  the  half  of  a  millieme.  If  the  first  addition  of  decimal 
salt  solution  fails  to  give  a  precipitate,  the  normal  solution  con- 
tains an  excess  of  salt,  and  2  c.c.  of  decimal  silver  solution  are 
now  added,  one  of  which  neutralizes  or  precipitates  the  1  c.c.  of 
decimal  salt  solution  added,  the  other  acting  on  the  excess  of  salt 
in  the  solution.  The  decimal  silver  solution  is  added  until  no 
further  cloud  appears,  in  the  same  way  as  described  for  the 
decimal  salt  solution.  In  this  way  the  exact  strength  of  the 
normal  salt  solution  is  determined  in  duplicate.  If  it  is  incorrect 
to  the  extent  of  more  than  2  points  fineness  either  way  (i.e.,  either 
strong  or  weak),  it  is  corrected  by  the  addition  of  either  water 
or  salt,  and  restandardized,  and,  when  correct,  a  new  decimal 
salt  solution  made  up  from  it.  Its  strength  is  finally  recorded 
on  the  bottle  as  follows:  100  c.c.  =  1000  mgs.  Ag,  or  whatever  it 
may  actually  be. 

The  Assay. — It  is  evident  from  the  preceding  that  the  amount 
of  bullion  to  be  taken  for  assay  must  contain  as  nearly  as  possible 
1000  mgs.  Ag  in  order  to  make  the  titration  with  solution  as 
short  as  possible,  and  avoid  undue  additions  of  the  decimal 
solutions.  For  this  reason  the  bullion  on  which  the  silver  deter- 
mination is  to  be  made  is  first  assayed  by  the  cupellation  method, 


180  A    MANUAL    OF    FIRE    ASSAYING 

or  at  least  a  preliminary  assay,  described  under  this  method,  is 
made,  and  from  these  data  the  amount  of  bullion  containing 
1000  mgs.  of  silver  calculated.     For  instance,  suppose  the  cupella- 
tion  method  shows  the  bullion  to  be  900  fine  in  silver,  then 
900          :    '       1000  ::         1000       :  x. 

fineness  :  amt.  of  bullion ::  silver  :  amt.  of  bullion, 
or  1111.11  mgs.  bullion  contains  1000  mgs.  Ag.  This  amount  of 
bullion  is  then  weighed  out  in  duplicate  and  dissolved  in  acid, 
placed  in  titration  bottles,  as  described  above,  under  "  Standardi- 
zation of  Solutions, "  and  titrated. 

The  calculation  for  fineness  is  as  follows:  Suppose  the  strength 
of  the  normal  solution  is  100  c.c.  =  1001  mgs.  Ag,  and  that  99.8  c.c. 
of  normal  solution  were  used  in  the  titration  (100  c.c.  normal 
salt,  and  2  C.c.  decimal  silver) ;  then 

100     :     1001     ::     99.8     :       x 

the  x,  or  amount  of  silver  in  bullion,  equaling  998.99  mgs.;  and 
the  fineness  is 

1111.11     :     998.99     ::     1000     :       y 
the  y,  or  fineness,  equaling  899.1. 

The  only  metal  interfering  with  the  salt  titration  is  mercury, 
which  will  be  precipitated  by  the  NaCl  as  Hg2Cl2;  the  addition  of 
20  c.c.  sodium  acetate  and  a  little  free  acetic  acid  to  the  assay 
will  prevent  the  precipitation  of  the  mercury.  Mercury  can  be 
detected  in  the  titration  if  the  AgCl  has  not  turned  dark  as  the 
result  of  exposure  to  sunlight.  Mercury  will  be  found  sometimes 
in  mill  bullions  which  have  been  retorted  at  too  low  a  temperature. 
The  assay  and  standardization  of  the  solution  should  be  carried 
out  where  there  is  no  sun,  and  where  light  is  not  too  strong. 

THE  ASSAY  OF  GOLD  BULLION  FOR  SILVER  BY  A  WET 
METHOD. — The  accurate  estimation  of  silver  in  bullions  contain- 
ing a  large  proportion  of  gold  is  not  all  that  can  be  desired  by  the 
ordinary  fire  method.  The  Gay-Lussac  method  is  generally 
not  applicable  on  account  of  the  large  amount  of  bullion  that 
must  be  taken  for  a  sample  in  order  to  get  1  gram  of  silver. 
The  foUowing  wet  method1  will  yield  good  results.  Take  0.5 
gram  of  the  bullion,  fuse  with  1.5  gm.  of  pure  cadmium  under 
a  cover  of  potassium  cyanide  in  a  porcelain  crucible  in  the 
flame  of  a  blast  lamp.  Enough  cyanide  must  be  used  to 

1  E.  H.  Taylor,  Australian  Mining  Standard,  August  26,  1908,  235.  Consult  also  J.  E. 
Clennel.  Eng.  and  Min.  Jour.,  LXXXIII,  1099. 


THE   ASSAY    OF    BULLION  181 

cover  the  cadmium.  Five  minutes  is  sufficient  to  insure  fusion. 
Allow  to  cool,  place  in  stream  of  running  water  which  will 
rapidly  dissolve  the  cyanide  and  leave  the  alloy.  Transfer 
this  to  a  flask  with  20  c.c.  of  water,  add  40  c.c.  of  HNO3  in 
installments  of  10  c.c.  each  while  boiling  for  one  hour.  Dilute 
to  150  c.c.  and  add  10  c.c.  of  ferric  alum  indicator  and  titrate 
with  the  standard  solution  of  NH4CNS.  This  solution  is  made 
as  follows:  1.6  grams  of  pure  NH4CNS  are  dissolved  in  1000 
c.c.  of  distilled  water.  This  is  standardized  against  pure  sil- 
ver foil  dissolved  in  HNO3  and  diluted  to  150  c.c.  1  c.c.  of 
the  solution  equals  approximately  4.483  parts  of  Ag  per  1000 
under  the  conditions  described  above.  The  indicator  is  a  sat- 
urated solution  of  ferric  alum.  The  appearance  of  the  red  color 
marks  the  end  point.  Copper  in  amounts  of  100  parts  per  1000 
in  the  bullion  does  not  interfere  with  the  delicacy  of  the  end 
point.  In  case  the  bullion  is  very  high  in  gold  the  cadmium 
must  be  increased.  The  parted  gold  is  recovered  from  the 
residues  in  the  flasks. 

THE  ASSAY  OF  GOLD  BULLION.  1.  Sampling.— Bullion  bars 
and  retort  sponge,  as  shipped  to  the  United  States  assay  offices 
and  mints,  is  remelted  into  bars  to  make  the  deposit  uniform. 
These  are  sampled  by  taking  chips  from  diagonally  opposite 
corners,  each  of  which  is  rolled  into  a  fillet  and  assayed  by  different 
assayers,  who  are  required  to  check  with  each  other  within  narrow- 
limits;  if  they  do  not,  the  bar  is  remelted,  stirred  thoroughly,  and 
recast;  then  sampled  again  and  assayed.  If  base  bullion,  or  one 
which  liquates  seriously  on  cooling,  is  to  be  assayed,  dip-samples 
are  taken  from  the  molten  bullion  by  means  of  a  small  graphite 
ladle,  and  the  sample  granulated  in  warm  water.  Silver  bullion 
is  sampled  in  the  same  manner. 

2.  Preliminary  Assay. — This  is  made  in  the  way  described 
for  silver  bullion,  except  that  in  the  assay  of  gold  bullion  no 
determination  of  silver  is  made  by  cupellation;  but  if  this  is  to 
be  determined,  the  mint  wet  method  is  used.  Experienced 
assayers  can  judge  the  approximate  fineness  of  gold  bullion  by 
the  color,  and  add  the  proper  amount  of  silver  necessary  to  insure 
parting.  In  the  San  Francisco  mint,  2  parts  of  Ag  to  1  of  Au 
are  used.1  The  British  royal  mint  formerly  used  2.75  parts  of 
Ag.  to  1  of  Au,2  but  now  uses  2  to  1.  More  than  3  parts  Ag 

1  John  W.  Pack,  "Assaying  of  Gold  and  Silver  in  U.  S.  Mint",  in  M in.  and  Sci.  Press, 
LXXXVII,  317. 

2  Rose,  in  Eng.  and  Min.  Jour.,  LXXX,  492. 


182  A    MANUAL    OF    FIRE   ASSAYING 

to  1  of  Au  should  not  be  used,  otherwise  the  "cornet"  of  gold  is 
apt  to  break  up.  With  less  than  2  parts  of  Ag;  too  much  Ag 
is  retained,  although  with  continued  boiling  1.75  parts  Ag  will 
part  Au  from  Ag.1  For  the  preliminary  assay,  500  mgs.  (1000 
milliemes)  are  weighed  out,  silver  added  according  to  judg- 


FIG.  59. — JEWELERS'  ROLLS. 

ment  to  bring  the  ratio  of  silver  to  gold  to  2  or  2.5  (allowing 
for  silver  in  the  alloy),  and  the  bullion  and  silver  wrapped  in 
10  grams  sheet  lead  and  cupelled  at  850°  C. 

The  resultant  bead  is  cleaned,  weighed,  flattened  and  rolled 
out  in  jeweler's  rolls  to  a  fillet  of  the  approximate  thickness  of  a 
visiting  card.  If  some  copper  is  present  in  the  bullion,  enough 

1  Rose,  "Metallurgy  of  Gold,"  p.  493. 


THE   ASSAY    OF    BULLION  183 

is  retained  by  the  gold  bead  to  toughen  it,  and  it  can  be  easily 
rolled  without  cracking,  if,  between  reductions  by  the  rolls,  the 
fillet  is  annealed  at  a  dull-red  heat.  The  presence  of  copper 
in  the  button  aids  in  the  total  removal  of  lead  during  the 
cupellation.1 

The  fillet  is  then  again  annealed  and  rolled  into  a  spiral, 
called  a  "cornet,"  and  parted  in  a  parting  flask.  This  is  filled 
with  30  c.c.  of  HNO3  sp.  gr.  1.20,  free  from  Cl,  H2SO4,  H2SO3, 
or  any  sulphide,  and  heated  to  boiling  (or  at  least  90°  C.)  for  20 
minutes.  The  acid  is  then  decanted  off,  and  the  cornet  washed 
carefully  several  times  with  hot  distilled  water  by  decantation. 
Then  30  c.c.  of  boiling  nitric  acid,  sp.  gr.  1.30,  are  added  to  the 
flask,  and  the  cornet  boiled  again  for  20  minutes,  after  which  the 
acid  is  decanted,  and  the  washing  with  hot  water  repeated. 
During  the  boiling,  a  parched  pea  added  to  the  flask  prevents 
bumping.  The  flask  is  now  filled  to  the  very  top  with  cold 
distilled  water,  a  suitably  sized  porcelain  parting-cup  placed  over 
the  mouth,  fitting  reasonably  tight,  and  the  flask  inverted.  The 
cornet  will  settle  into  the  parting-cup,  and  the  flask  is  then  gently 
tipped  to  permit  the  water  to  escape,  the  water  is  decanted  from 
the  parting-cup,  and  the  cornet  gently  dried.  When  dry,  the 
cornet  is  transferred  to  a  clay  annealing  cup,  the  cover  is  put 
on,  and  the  cup  is  placed  in  the  muffle,  and  the  cornet  annealed 
at  a  full-red  heat.  It  is  then  weighed.  The  weight  of  the  gold 
plus  that  of  the  added  silver,  subtracted  from  the  weight  of  the 
cupelled  bead,  gives  the  approximate  amount  of  silver  in  the 
assay.  This  added  to  the  weight  of  the  gold  and  subtracted 
from  500  mgs.  (the  weight  of  bullion  taken)  gives  the  approximate 
amount  of  base  metal.  If  the  amount  of  silver  added  to  part 
the  gold  has  raised  the  ratio  of  Ag  to  Au  over  3  to  1,  the  gold 
will  probably  have  broken  up,  or  at  least  parts  will  have  broken 
from  the  edges  of  the  cornet;  care  must,  in  this  case,  be  taken  to 
collect  all  of  it  in  the  washing.  If  the  results  show  that  the 
ratio  of  Ag  to  Au  has  been  less  than  2  to  1,  the  cornet  must  be 
recupelled  with  2.5  parts  Ag  and  parted  as  described. 

The  Assay. — The  final  assay  is  made  up  from  data  obtained 
in  the  preliminary  assay.  Duplicates  on  1000  milliemes  are  run, 
with  a  check  assay  made  up  in  composition  as  near  to  that  of 
the  bullion  as  possible,  as  described  for  the  cupellation  assay  of 
silver.  In  making  up  the  check,  proof  gold  and  proof  silver  are 

»  Rose,  "Refining  Gold  Bullion,"  in  Trans.  I.  M.  M.,  April  13,  1905. 


184 


A    MANUAL    OF    FIRE   ASSAYING 


used,  and  c.p.  copper  foil.  The  United  States  mints  use  various 
proof  alloys  in  the  making  up  of  check  assays.  For  the  assay  of 
fine  gold  bars  (990  fineness  and  above),  a  proof  alloy  of  1000 
gold,  2000  silver,  and  30  parts  copper  is  used.  For  coin  riletal 
(900  parts  fine),  a  proof  alloy  of  gold  900  parts,  silver  1800  parts, 
copper  100  parts  is  used.  For  the  determination  of  base  metal 
(the  difference  between  the  gold  and  silver,  and  the  500  mgs. 
taken  for  assay),  a  proof  alloy  of  gold  900  parts,  silver  90  parts, 
copper  10  parts  is  used.1  In  this  last  the  gold  need  not  be  proof 
gold,  but  may  be  remelted  cornets.  It  is  to  be  noted  that  these 
proof  alloys  are  made  up  on  the  assumption  that  2  parts  of  Ag 
to  1  of  gold  are  used  in  parting.  The  British  mint  uses  a  proof 
alloy,  or  trial  plate,  916.6  fine  in  gold. 

For  the  assay  of  crude  gold  bullion,  i.e.,  mill  bullion,  the  proof 
alloy  for  fine  gold  bars  is  generally  used. 

The  amount  of  lead  used  in  the  cupellation  is  as  follows:2 

TABLE  XL.— LEAD  RATIO  IN  CUPELLATION 


Amount  of  gold  per 


Amount  of  lead 


1000  parts 

Ratio  of  lead  to  copper 

(base  metal  present) 

Milligrams 

Grams 

916.6 

8 

96  to  1 

866 

9.15 

68  to  1 

770 

14.75 

64  to  1 

666 

16.00 

48  to  1 

546 

17.50 

38  to  1 

333 

18.00 

27  to  1 

To  the  duplicates  of  the  1000  milliemes  of  bullion,  the  proper 
amount  of  Ag  is  added,  to  bring  the  ratio  of  Ag  to  Au  to  2  to  1, 
and  then  they  are  wrapped  in  the  proper  amount  of  c.p.  sheet 
lead.  The  check  is  made  up  as  indicated  by  the  preliminary 
assay,  and  the  three  assays  cupelled  as  described  for  the  assay 
of  silver  bullion.  The  three  beads  are  then  treated  and  parted, 
as  described  for  the  preliminary  assay.  The  two  bullion  assays 
should  not  differ  by  more  than  0.25  part  of  a  millieme.  The 


1  John  Pack,  ibid. 

2  Rose.  "Metallurgy  of  Gold."  1902,  p.  494. 


THE    ASSAY    OF    BULLION  185 

correction  as  indicated  by  the  check  should  then  be  applied, 
whether  this  be  plus  or  minus.  The  difference  between  the  fine  . 
gold  in  the  check  and  that  obtained  by  the  assay  of  the  check  is 
the  surcharge,  which  is  more  definitely  defined  in  Chapter  XI, 
on  "  Errors  in  the  Assay  for  Gold  and  Silver."  This  surcharge 
will  usually  amount  to  about  0  for  a  bullion  of  about  700  to  800 
fine;  above  that  there  will  be  a  "plus  surcharge,"  and  below  that 
a  "  minus  surcharge."  The  plus  surcharge  will  be  subtracted  and 
the  minus  surcharge  added. 

THE  PREPARATION  OF  PROOF  GOLD.— This  is  prepared  by 
dissolving  practically  pure  gold  (cornets)  in  nitro-hydrochloric 
acid,  permitting  the  solution,  after  some  dilution,  to  stand  for  four 
days  to  allow  AgCl  to  settle  out.  It  is  then  decanted  very  care- 
fully by  siphoning.  The  gold  chloride  solution  is  then  evaporated 
almost  to  dryness,  taken  up  with  plenty  of  distilled  water,  a  few 
c.c.  of  NaBr  or  KBr  solution  added,  allowed  to  stand  for  some 
days,  and  again  decanted  by  siphoning,  after  which  operation  it 
is  slowly  dropped  from  a  burette  into  a  beaker  containing  c.p. 
aluminium  foil.  When  precipitation  is  complete,  HC1  is  added  to 
dissolve  the  excess  of  Al,  and  the  residual  gold  is  washed  thor- 
oughly with  water  by  decantation,  and  then  dried  and  melted 
into  a  bead  in  a  fresh  cupel  (but  not  cupelled  with  Pb) .  The  gold 
is  then  rolled  into  a  thin  strip  for  use.1 

Proof  silver  is  prepared  by  dissolving  c.p.  silver  foil  in  HNO3, 
and  then  precipitating  with  HC1  after  filtering.  The  AgCl  is 
thoroughly  washed  with  diluted  HC1  and  converted  into  metallic 
silver  by  Al  in  the  presence  of  HC1,  all  Al  being  dissolved  out. 
The  washed  silver  is  then  fused  in  a  porcelain  crucible,  and 
rolled  into  strips.2 

1  Consult  also  Rose,  "Metallurgy  of  Gold,"  p.  10,  and  Pack,  ibid. 

2  John  Pack,  ibid. 


CHAPTER  XIII 

THE  ASSAY  OF  ORES  AND  ALLOYS  CONTAINING  PLAT- 
INUM, IRIDIUM,  GOLD,  SILVER,  ETC. 

Materials  containing  some  of  the  above  elements  are  pre- 
sented to  the  assayer  for  determination  in  the  shape  of  sands 
containing  chiefly  platinum,  alloys  and  jewelers'  sweeps,  and, 
more  rarely  ores  containing  platinum  in  the  form  of  the  mineral 
sperrylite,  etc. 

The  assay  for  platinum  and  associated  metals  is  a  difficult 
one,  due  to  the  fact  that  in  the  parting  of  the  precious  metal 
beads,  by  acids,  complex  reactions  take  place,  by  which  platinum, 
palladium,  silver,  etc.,  both  go  into  solution  and  are  retained  in 
the  residue,  unless  certain  well  established  ratios  of  metals 
present  are  observed  and  the  parting  operation  repeated  several 
times.  The  alloys  of  platinum  and  silver  have  been  most  thor- 
oughly investigated  in  this  connection.1  When  the  alloy  is 
more  complex,  i.e.,  contains  also  gold,  palladium,  iridium, 
rhodium,  etc.,  the  difficulties  of  the  assay  are  increased;  the  data 
at  present  available  are  meager. 

Platinum  nuggets  from  the  Urals  contain:2  Pt,  60  to  86.5  per 
cent.;  Fe,  up  to  19.5  per  cent.;  Ir,  up  to  5  per  cent.;  Rh,  up  to 
4  per  cent.;  Pd,  up  to  2  per  cent.;  also  Os,  Ru,  Cu,  Au,  and  iri- 
dosmium. 

When  material  containing  Au,  Ag,  Pt,  Pd,  Ir,  Rh,  Ru,  Os,  and 
IrOs  is  fused  by  the  crucible  assay  or  melted  with  lead,  the 
Au,  Ag,  Pt,  Pd,  Ir,  Rh,  IrOs  are  collected  by  the  lead  and  the 
Ru,  and  Os  only  partially  so.  If  the  resultant  lead  button  is 
cupelled,  the  final  bead  will  contain  the  Au,  Ag,3  Pt,  Pd,  Ir,  Rh, 
IrOs,  and  a  comparatively  small  portion  of  the  Os  and  Ru,  the 
most  of  these  two  metals  being  lost  by  oxidation.  The  presence 
of  any  considerable  amounts  of  Os  and  Ru  in  the  lead  button, 

1  Thompson  and  Miller,  in  Jour.  Am.  Chem.  Soc.,  XXVIII,   1115.     See  this  paper  for 
other  references. 

2  Kemp,  in  Eng.  and  Min.  Jour.,  LXXIII,  513   (Notes  on  Platinum  and  Associated 
Metals). 

3  Exclusive  of  losses  by  absorption  and  volatilization. 

186 


THE   ASSAY    OF    ORES   AND   ALLOYS  187 

owing  to  the  fact  that  they  will  not  alloy  readily,  causes  them  to 
appear  as  a  black  scum  or  as  spots  on  the  bead,  near  the  end  of 
the  cupellation.  The  presence  of  the  platinum  group  of  metals, 
raising  the  melting-point  of  the  gold-silver  alloy,  renders  neces- 
sary a  high  temperature  of  cupellation  in  order  to  remove  lead. 
Even  then,  when  the  ratio  of  Ag  to  Pt,  etc.,  is  less  than  5  to  1, 
lead  will  be  retained  in  varying  proportions  at  the  cupellation 
temperature  of  gold  bullion.  To  get  rid  of  the  lead,  the  propor- 
tion should  be  10  to  I.1  The  following  points  on  the  first  cupella- 
tion of  the  lead  buttons,  resulting  from  the  assay  of  material 
containing  Pt,  etc.,  will  give  the  assayer  an  idea  of  what  is 
present.  When  Pt  alone,  or  with  very  little  silver  is  present,  the 
bead  from  the  cupellation  (at  a  comparatively  high  temperature) 
is  rough,  dull  gray,  flat,  and  contains  lead. 

If  more  silver  is  present,  but  less  than  2  parts  of  Ag  to  1  of 
Pt,  the  beads  are  rough,  flat,  and  have  a  crystalline  surface. 

If  more  than  2  parts  of  Ag  are  present  and  not  more  than  15, 
the  bead  approaches  more  nearly  the  appearance  of  a  normal 
silver  bead,  but  has  a  more  steely  appearance  and  is  flatter  in 
proportion  to  the  Pt,  etc.,  contained. 

Beads  containing  more  platinum  than  1  in  16  will  not  blick 
or  flash.2 

The  effect  on  the  appearance  of  the  bead  of  Pd,  Rh,  Ir  is  similar 
to  that  of  Pt,  but  not  identical. 

Owing  to  the  difficulty  in  alloying  iridium,  this,  when  present, 
is  apt  to  be  found  at  the  bottom  of  the  bead,  in  the  shape  of 
fine  black  crystalline  particles.3 

THE  ACTION  OF  ACID  ON  THE  ALLOY  BEADS.— A  great  deal 
of  literature  exists  on  this  point;  but  most  of  it  is  very  conflict- 
ing; some  facts,  however,  have  been  definitely  established. 

Nitric  Acid. — In  an  alloy  of  Pt  and  Ag  treated  by  HNO3, 
platinum  goes  into  solution  in  various  proportions,  depending 
on  the  ratio  of  Ag  to  Pt,  and  probably  to  some  extent  on  the 
strength  of  acid.  It  has  been  stated  that  when  the  ratio  of 
Ag  to  Pt  is  12  or  15  to  1,  this  solution  of  Pt  is  complete  in  one 
treatment,  but  this  has  been  disproved  by  later  investigation.4 
In  order  to  accomplish  the  solution  of  Pt,  the  acid  treatment 

1  Sharwood,  "Cupellation  on  Platinum  Alloys,  containing  Ag  and  Au,"  in  Jour.  Soc. 
Chem.  Ind.,  XXIII,  No.  8. 

2  Schiffner,  in  Min.  Ind.,  VIII,  397. 

3  Rose,  "Metallurgy  of  Gold,"  p.  514. 

4  Thompson  and  Miller,  in  Jour.  Am.  Chem.  Soc.,  XXVIII.  115. 


188  A   MANUAL    OF    FIRE    ASSAYING 

must  be  repeated  at  least  once  or  twice,  with  a  possible  recupel- 
lation  of  the  residue  with  silver  before  the  second  treatment. 
It  is  even  then  doubtful  if  all  of  the  Pt  can  be  dissolved.  The 
Pt  goes  into  solution  in  the  nitric  acid  in  colloidal  form,  giving 
a  brown  to  blackish  color  to  the  solution.  When  gold  is  present 
in  the  silver-platinum  alloy,  the  solubility  of  the  Pt  seems  to 
be  decreased,1  unless  the  ratio  of  Pt  to  Au  to  Ag  is  1:2:15,2 
when  most,  but  not  all,  of  the  Pt  and  all  the  Ag  go  into  solu- 
tion. Palladium  goes  into  solution  with  nitric  acid  when  at  least 
3  parts  of  Ag  to  1  of  Pd  are  present,3  yielding  an  orange-colored 
solution;  but  double  parting  is  necessary  to  insure  complete  so- 
lution. (This  point  is  not  sufficiently  established.4)  The  orange- 
colored  solution  indicates  colloidal  palladium. 

Iridium  and  Rhodium. — Iridium  present  in  the  beads  is 
unacted  upon  by  HNO3  and  remains  with  the  gold.5  Rhodium 
is  slightly  dissolved,  but  most  of  it  remains  with  the  gold.  Iridos- 
mium  is  not  dissolved.  Osmium  is  dissolved.  Ruthenium  is 
not  dissolved. 

Sulphuric  Acid. — Platinum,  alloyed  with  silver  and  gold, 
can  be  separated  from  the  silver  and  remains  with  the  gold,  if 
concentrated  sulphuric  acid  is  used  in  parting.  In  order  to 
insure  thorough  parting,  at  least  10  parts  of  silver  to  1  part  Pt 
and  gold  should  be  present,  and  double  parting  resorted  to, 
otherwise  silver  will  remain  with  the  residue.6  The  parting  in 
H2SO4  leaves  the  Pt  and  gold  in  a  very  fine  state  of  division  (but 
not  as  a  colloid),  some  of  which  is  very  apt  to  be  lost  in  decanting, 
so  that  it  is  best  to  separate  by  filtering  through  an  ashless  filter. 
It  is  also  to  be  noted  that  lead  may  be  present  in  consequence  of 
too  low  a  cupellation  temperature,  in  which  case  the  residue  should 
be  treated  with  ammonium  acetate,  to  remove  lead  sulphate. 

Palladium. — In  parting  with  H2SO4  this  goes  into  solution 
with  the  silver,  giving  an  orange-colored  solution.  Whether  this 
solution  is  complete,  has  not  as  yet  been  demonstrated.7 

Ir,  IrOs,  Rh,  and  Os  and  Ru  in  the  bead  are  not  dissolved. 

Nitro-Hydrochloric  Acid. — From  the  residue  of  the  sulphuric 
acid  parting,  the  Pt,  Au,  and  any  Pd  left  is  dissolved  by  dilute 

1  Sharwood,  ibid. 

2  Lodge,  "Notes  on  Assaying,"  p.  215. 

3  Rose,  "Metallurgy  of  Gold,"  p.  514. 

4  Lodge,  "Notes  on  Assaying,"  pp.  218,  219. 
s  Rose,  ibid. 

6  Thompson  and  Miller,  ibid. 

7  Lodge  holds  the  contrary,  p.  219. 


THE    ASSAY    OF    ORES    AND    ALLOYS  189 

aqua  regia,  1  to  5,  leaving  Ir,  IrOs,  and  Rh,  and  some  Ru  and  Os, 
if  present.  This  last  residue,  treated  with  strong  aqua  regia, 
removes  Ir,  leaving  iridosmium  and  rhodium  as  a  final  residue. 

METHODS  OF  ASSAY.  1.  Ores.— Rich  ores,  carrying  Ft,  etc., 
in  grains,  present  difficulty  in  sampling,  inherent  to  any  ore 
containing  "  metallics. "  It  is  best  to  take  from  30  to  50  grams 
of  the  sample  and  fuse  it  with  6  times  its  weight  of  lead  in  a 
crucible,  fluxing  the  gangue.  The  lead  is  poured,  and  after 
cooling  the  slag  is  detached  carefully,  the  lead  platinum  alloy 
being  brittle,  weighed  and  remelted  under  charcoal  in  order  to 
insure  a  uniform  alloy,  and  then  granulated  as  fine  as  possible 
by  pouring  into  a  large  volume  of  cold  water  from  a  considerable 
height.  The  resultant  sample  is  then  dried  and  is  ready  for 
assay.  An  amount  containing  approximately  200  mgs.  Pt  is 
weighed  out  and  scorified  with  50  grams  Pb  into  a  20-gram 
button. 

If,  in  the  low-grade  ores,  the  Pt,  etc.,  is  present  as  grains,  a 
weighed  quantity  is  concentrated  by  panning  and  the  concen- 
trates scorified  with  20  to  25  times  their  weight  of  test  lead,  and 
the  button  treated  according  to  method  No.  1  or  2,  as  below. 
If  the  ore  contains  the  rare  metal  in  other  form,  crucible  fusions 
are  made  on  1  assay  ton,  as  with  gold  and  silver  ores,  and  if  very 
low  grade,  the  buttons  from  4  to  5  fusions  are  scorified  into  one 
button,  final  duplicates  being  made  as  usual.  The  lead  buttons 
are  treated  as  below. 

2.  Alloys. — An  amount  of  drillings  or  filings  (representing 
a  true  sample  of  the  alloy),. containing,  if  possible,  not  to  exceed 
200  mgs.  of  Pt,  etc.,  is  weighed  out  and  scorified  with  80  grams 
of  test  lead,  to  a  button  of  about  18  to  20  grams.  The  lead 
buttons  are  treated  as  outlined  below. 

First  Method. — The  lead  button  obtained  by  any  of  the 
foregoing  methods  is  cupelled  at  a  temperature  of  at  least  900°  C., 
or,  better,  950°  C.,  and  the  resultant  bead  examined.  If,  from 
the  foregoing  description  of  the  appearances  of  a  bead,  it  is 
thought  that  the  ratio  Ag  to  Pt,  Au,  etc.,  is  less  than  10  to  1, 
the  button  is  removed,  the  necessary  silver  added  to  bring  it  up 
to  the  above  ratio,  recupelled  with  5  to  8  grams  of  lead  at  a  tem- 
perature of  900°  C.,  and  weighed.  The  bead  is  then  flattened 
and  rolled  out  into  a  cornet,  if  large  and  not  too  brittle,  and 
parted  with  15  c.c.  H2S04  concentrated,  boiling  for  15  to  20 
minutes.  The  acid  is  then  decanted  into  a  beaker  and  saved, 


190  A   MANUAL    OF    FIRE    ASSAYING 

the  residue  re-treated  with  5  c.c.  more  of  acid  for  10  minutes, 
and  the  residue  and  acid  washed  into  the  beaker  containing 
the  first  acid.  The  acid  is  then  diluted  and  the  residue 
separated  by  nitration  through  a  small  ashless  filter,  and  thor- 
oughly washed  with  hot  water  to  insure  removal  of  Ag2SO4. 
The  filter-paper  is  dried  and  carefully  transferred  to  a  porcelain 
parting-cup  or  an  annealing  cup,  and  the  carbon  burnt  off  in 
the  muffle.  The  annealed  residue  is  brushed  out  on  the  scale 
pan  of  the  bead  balance  and  weighed.  It  consists  of  gold,  plati- 
num, indium,  iridosmium,  rhodium,  and  possibly  osmium  and 
Ru  (if  any  escaped  oxidation  during  the  cupellation) ,  and  perhaps 
some  palladium.  Its  color  will  be  gray  or  black,  if  the  rare 
metals  are  present  to  any  extent.  If  not,  the  characteristic  gold 
color  will  show.  The  palladium  is  largely  in  the  filtrate.  (It  is 
questionable  how  complete  this  solution  is.1)  If  it  has  been 
unnecessary  to  add  Ag  to  the  cupellation  to  get  the  10  to  1  ratio, 
the  difference  in  weight  between  the  original  bead  and  the  weight 
of  the  residue  represents  the  Ag.  If  silver  had  to  be  added  and 
the  bead  recupelled,  the  weight  of  the  added  silver  plus  that  of 
the  residue,  subtracted  from  the  weight  of  the  recupelled  bead, 
gives  the  silver.  Allowance  must,  however,  be  made  for  con- 
siderable loss  of  silver  as  a  result  of  high  cupellation  temperature. 
If  accurate  silver  results  are  required,  a  duplicate  assay  on  the 
material  must  be  run,  and  the  silver  requisite  to  bring  the  ratio 
up  to  10  to  1  is  added  at  once  to  the  lead  button,  one  cupellation 
only  being  made.  At  the  same  time  this  is  run,  a  check  assay 
is  run  beside  it,  made  up  of  the  same  weight  of  lead,  and  the 
proper  weight  of  silver,  i.e.,  the  amount  added  to  the  first  cupel- 
lation plus  the  amount  approximately  known  to  be  in  the  assay. 
The  loss  in  this  will  give  the  correction  to  be  added  to  the  assay 
for  Ag.  It  may  be  desirable  to  determine  Ag  in  the  wet  way. 
(See  "  The  Assay  of  Silver  Bullion. ") 

The  residue  is  now  wrapped  in  8  to  10  grams  of  lead  foil  with 
at  least  20  times  its  weight  in  silver  and  cupelled  again  at  a 
high  temperature.  The  bead,  if  large,  is  rolled  out  and  heated 
to  boiling  in  a  mattrass  or  flask  for  20  minutes  with  HNO3, 
sp.  gr.  1.20,  after  which  the  acid  is  decanted  into  a  beaker,  and 
the  treatment  repeated  with  HNO3  of  1.26  sp.  gr.  The  residue, 
if  finely  divided,  should  now  be  filtered  through  an  ashless  filter 
and  washed  as  already  described.  If  not,  the  filtrate  can  be 

1  Ricketts  and  Miller,  in  "Notes  on  Assaying,"  state  that  the  Pd  dissolves  with  the  Ag. 


THE  ASSAY    OF    ORES  AND  ALLOYS  191 

decanted  and  the  residue  washed.  The  residue  consists  of  Au, 
Ir  and  iridosmium,  and  some  Rh  and  Ru.  If  there  is  a  sus- 
picion that  -any  platinum,  etc.,  remains,  the  residue  must  be 
re-treated  with  acid  until  of  constant  weight.  The  platinum  is 
in  the  filtrate,  which  will  be  colored  brown  or  black. 

The  difference  between  the  weights  of  the  first  and  second 
residues  is  platinum,  the  result  possibly  being  somewhat  high  if 
palladium  is  present  in  the  material  assayed.  The  second  residue 
is  now  warmed  in  a  mattrass  with  dilute  aqua  regia1  (1  to  5) 
for  15  minutes.  This  dissolves  the  gold,  some  of  the  Ru  and 
very  little  Rh,  leaving  the  Ir,  iridosmium  and  Rh,  with  some 
Ru.  The  residue  is  either  filtered  or  decanted,  as  necessary, 
dried,  annealed,  and  weighed.  The  difference  in  weight  between 
the  second  and  third  residues  represents  gold,  somewhat  high,  if 
the  Ru  has  partly  escaped  oxidation  and  volatilization  during 
cupellation.  The  gold  can  be  recovered  by  precipitation  with 
oxalic  acid,  as  described  in  the  second  method. 

If  the  third  residue  is  treated  with  strong  aqua  regia,  and 
boiled,  it  dissolves  out  the  iridium,  leaving  as  a  residue  the 
iridosmium  and  most  of  the  Rh.  This  is  dried,  annealed,  and 
weighed,  the  difference  in  weight  between  the  third  and  fourth 
residues  representing  iridium,  and  the  weight  of  the  fourth  residue 
representing  iridosmium  and  Rh.  The  method  determines  Ag, 
Pt,  Au,  Ir,  and  iridosmium  plus  Rh.  The  probable  errors  in 
the  determination  have  been  pointed  out.  Palladium  can  be 
satisfactorily  determined  only  by  wet  analysis. 

Second  Method.2 — Take  the  lead  button  from  the  ore  or 
alloy  assay,  and  scorify  at  a  high  heat,  with  additional  test  lead, 
if  necessary,  to  a  weight  of  8  to  10  grams.  It  should  contain 
less  than  5  per  cent.  Pt.  etc.,  in  order  to  be  malleable.  Roll 
out  the  button  into  a  long  thin  fillet  and  place  in  a  large  beaker 
with  200  c.c.  of  HNO3,  sp.  gr.  1.08,3  and  heat  until  all  action  ceases. 
Filter  through  a  small  ashless  filter  and  wash  the  residue  with 
hot  water.  Dry  the  residue  and  paper,  transfer  to  a  large-size 
parting-cup  and  ignite  in  the  muffle,  to  burn  off  the  carbon,  and 
oxidize  any  Pb  not  dissolved.  Then  heat  to  boiling  in  the  cup 
with  HN03,  1.08  sp.  gr.,  decant,  wash  thoroughly  with  hot  water, 
dry,  anneal,  and  weigh  the  residue.  This  consists  of  Au,  Pt,  Ir, 
iridosmium,  and  most  of  the  Rh,  as  well  as  the  Ru  and  Os  which 

1  Concentrated  aqua  regia  is  1  part  HNOj,  sp.  gr.  1.42,  and  3  parts  HC1,  sp.  gr.  1.20. 

3  E.  H.  Miller,  in  School  of  Mines  Quart.,  XVII,  26. 

3  81  parts  distilled  H2O  to  19  parts  HNOj  cone.  (sp.  gr.  1.42). 


192  A   MANUAL    OF    FIRE    ASSAYING 

escaped  oxidation  and  volatilization  during  the  scorification. 
The  filtrate  contains  the  Ag  and  Pd  and  a  little  of  the  Rh. 

Replace  the  residue  in  the  capsule  and  warm  (not  boil)  with 
dilute  aqua  regia  (1  to  5)  for  10  minutes.  This  dissolves  the 
Au  and  Pt.  Decant  the  solution  into  a  small  beaker,  wash  the 
residue,  dry,  anneal,  and  weigh.  The  second  residue  consists  of 
Ir,  IrOs,  Rh,  and  a  little  Os  and  Ru.  This  residue  is  boiled 
with  strong  aqua  regia,  which  dissolves  the  Ir  and  some  Os  and 
Ru,  and  leaves  in  the  third  residue  the  IrOs  and  Rh,  with  a  little 
Os  and  Ru.  This  is  washed,  decanted,  and  weighed  as  before. 
The  nitrate  from  the  treatment  of  the  first  residue,  which  con- 
tains the  gold,  is  evaporated  just  to  dryness,  but  not  baked,  so 
as  to  prevent  reduction  of  the  gold  chloride,  taken  up  with  dis- 
tilled water  and  a  drop  of  HC1,  and  the  gold  in  it  precipitated  by 
warming  with  crystals  of  oxalic  acid  for  a  half  hour,  filtering, 
and  drying  the  yellow  coherent  precipitate  of  gold.  This  is 
transferred,  filter-paper  and  all,  to  a  piece  of  sheet  lead,  silver 
added  to  the  weight  of  3  times  the  gold  present,  approximately, 
and  cupelled,  the  bead  being  parted  in  HNO3  as  usual  and  the 
gold  annealed  and  weighed.  The  weight  of  the  gold,  subtracted 
from  the  difference  in  weight  between  the  first  and  second  residues 
is  the  platinum.  This  last  may  also  be  estimated  by  destroying 
the  oxalic  acid  in  the  filtrate  from  the  separation  of  gold,  and  pre- 
cipitating as  (NH4)2  PtCle.1 

It  is  to  be  noted  that,  by  the  assay  as  outlined,  neither  osmium 
nor  ruthenium  can  be  determined,  owing  to  their  volatility  during 
part  of  the  operation;  that  palladium  cannot  be  readily  deter- 
mined, owing  to  its  varying  solubility;  and  that  when  rhodium 
or  the  above  metals  are  present  in  any  appreciable  quantity, 
some  of  the  results  obtained  are  liable  to  error.  Rhodium, 
osmium,  and  ruthenium  are  among  the  rarer  of  the  group,  and 
are  frequently  absent.  The  methods  outlined  will  serve  to 
determine  reasonably  well  platinum,  gold,  silver,  iridium,  and 
iridosmium  plus  rhodium.  When  the  other  elements  of  the  group 
are  present,  wet  methods,  not  within  the  scope  of  this  book, 
must  be  resorted  to. 

In  the  ordinary  assay,  as  carried  out  for  gold  and  silver, 
platinum  and  palladium  may  escape  the  assayer  if  present  in 
only  small  quantities,  for  obvious  reasons.  Parting  in  sulphuric 
acid  is  therefore  necessary  to  determine  whether  they  are  present.2 

1  Crookes,  "Select  Methods." 

3  An  orange-colored  solution  indicates  palladium. 


CHAPTER  XIV 

THE  ASSAY  OF  TIN,  MERCURY,  LEAD,  BISMUTH  AND 
ANTIMONY 

The  assay  of  ores  for  base  metal  by  fusion  is  still  carried  out  in 
practice,  especially  for  lead  and  tin.  The  fire  assay  gives,  not 
the  correct  metal  content,  but  the  yield  obtainable  in  smelting, 
although  in  metallurgic  operations  the  yield  may  be  greater  or 
less.  The  smelter,  therefore,  purchases  lead,  tin,  and  copper 
ores  on  the  basis  of  the  "dry"  or  fire  assay.  The  fire  assay  of 
copper  is  practically  no  longer  in  use,  except  in  part  of  the  Lake 
Superior  district,  on  metallic  copper  concentrates,  and  in  pur- 
chasing copper  ores  the  assay  is  made  by  the  standard  electro- 
lytic method,  or  a  volumetric  method,  and  a  percentage  of 
from  1  to  1.5  deducted  to  indicate  dry  assay.  The  usual  deduc- 
tion is  1.3  per  cent.  Thus  the  dry  assay  of  copper  on  an  ore  is 
equivalent  to  the  percentage  obtained  by  the  electrolytic  method 
less  1.3  per  cent. 

While  wet  methods,  with  a  deduction,  will  in  all  probability 
be  employed  eventually  for  all  lead  ores,  as  it  is  now  for  impure 
lead  ores,  pure  lead  ores  are  still  assayed  by  the  fire  method. 
Tin  ores  are  almost  invariably  assayed  by  the  fire  method,  as  the 
wet  analysis  of  tin  is  long  and  tedious. 

THE  ASSAY  OF  TIN  ORES.— The  fire  assay  of  tin  ores  is  appli- 
cable only  to  those  ores  in  which  tin  exists  as  cassiterite,  the 
oxide  (Sn02).  The  chief  reasons  for  inaccuracies  in  the  fire  assay 
of  tin  are: 

1.  Some  of  the  tin,  reduced  in  the  assay  from  the  oxide,  is 
apt  to  be  volatilized  at  the  temperatures  necessarily  employed. 

2.  Metallic  tin  may  be  slagged  by  alkaline  carbonates  used  in 
some  of  the  methods  of  assay,  forming  stannates. 

3.  Foreign  metals  present  in  the  ore  are  apt  to  be  reduced 
and  enter  the  button. 

4.  Sulphides  present  carry  tin  into  the  slag.     If  sulphates 
are  present,  they  are  reduced  to  sulphides. 

5.  Silica  and  silicates,  always  present  in  the  ore,  even  after 
very  careful  concentration,  carry  tin  into  the  slag,  as  silicate, 

13  193 


194  A    MANUAL    OF    FIRE    ASSAYING 

while  the  SnO2  passes  through  the  lower  stage  of  oxidation  in 
being  reduced  to  metallic  tin, 

6.  The  cassiterite,  before  reduction,  is  apt  to  combine  with 
basic  fluxes  present  in  the  assay,  and  be  carried  into  the  slag  as 
stannates. 

From  this,  therefore,  it  is  evident  that  the  fire  assay  for  tin  is 
only  an  approximation,  although  in  many  cases  a  very  close  one. 
If  the  result  on  a  tin  ore  by  the  fire  method  checks  that  of  the 
standard  wet  method  (the  modified  Rose  method1),  it  is  to  be 
ascribed  to  a  balancing  of  errors,  due  to  the  presence  of  other 
metals  in  the  ore,  which  have  been  reduced  into  the  tin  button. 

Preparation  of  the  Ore  for  Assay. — It  is  essential  to  remove 
all  the  gangue  of  the  ore  and  have  for  the  assay  nothing  but  the 
cassiterite,  as  far  as  this  is  possible.  The  ore  is  roughly  crushed 
on  a  buck  board  and  put  through  a  40-mesh  screen,  crushings 
and  screenings  succeeding  each  other  at  frequent  intervals  in 
order  to  avoid  the  "sliming"  of  the  cassiterite.  If  the  ore  is 
low-grade,  i.e.,  below  2  per  cent.  Sn,  1000  grams  of  the 
crushed  ore  is  weighed  out  and  carefully  panned  in  a  gold  pan, 
the  first  pannings  being  saved  for  repanning.  The  ore  is 
concentrated  just  as  much  as  possible  without  incurring  loss 
of  cassiterite.  The  concentrates  from  the  repanning  of  the 
tailings  of  the  first  treatment  are  added  to  the  main  lot  of  con- 
centrates. Some  or  all  of  these  will,  unless  the  ore  is  very  pure, 
contain  probably  garnets,  feldspar,  tourmaline,  magnetite, 
zircons,  wolframite,  columbite,  sulphides,  quartz,  etc.  The 
concentrates  are  carefully  transferred  to  a  porcelain  dish,  dried, 
and  roasted  at  a  bright-red  heat  in  order  to  decompose  sulphides 
and  sulphates.  While  the  concentrates  are  still  red-hot,  they 
are  transferred  into  a  beaker  containing  water  in  order  to  make 
garnet  and  other  silicates  soluble  (all  except  uvarovite),  and 
after  decanting  water,  treated  with  nitro-hydrochloric  acid  to 
remove  most  of  the  contaminating  minerals,  except  quartz, 
wolframite,  and  some  garnet.  The  concentrates  are  then  fil- 
tered off  and  dried.  If  quartz  is  present,  this  can  be  removed  by 
transferring  the  filtered  concentrates  to  a  platinum  dish  and 
treating  with  HF.  This,  however,  will  rarely  be  necessary.  The 
concentrates  are  then  crushed  in  an  agate  mortar  to  pass  a  100- 
mesh  screen  and  treated  as  described  below. 

The  Assay. — The  two  best  methods  for  assay  are  the  cyanide 

»  Hofman,  "The  Dry  Assay  of  Tin  Ores,"  in  Trans.  A.  I.  M.  E.,  XVIII,  1. 


THE    ASSAY    OF    TIN,    MERCURY,    LEAD,    ETC.  195 

fusion  and  the  German  method,  with  black  flux  substitute. 
Of  these  two,  the  cyanide  fusion  is  generally  to  be  preferred,  as 
any  minerals  still  left  in  the  cassiterite  have  less  influence  on  the 
assay,  and  the  loss  of  tin  by  volatilization  is  reduced  to  a  mini- 
mum, on  account  of  the  low  temperature  employed. 

The  Cyanide  Fusion.1 — It  is  essential  to  use  only  the  purest 
cyanide  obtainable — the  best  sodium  or  potassium  cyanide  on 
the  market  for  use  in  the  cyanide  process.  Such  impurities  as 
K2CO3,  sulphates,  and  sulphides  in  cyanide  cause  serious  losses  in 
the  assay.  The  best  alkaline  cyanide  to  use  is  sodium  cyanide, 
which  may  readily  be  procured  at  the  present  time.  Some  of 
the  ordinary  commercial  cyanide  known  as  "potassium  cyanide" 
fuses  at  such  a  low  temperature  that  the  concentrates  sink  to 
the  bottom  of  the  crucible  before  reduction,  and  when  reduction 
finally  takes  place  the  little  globules  of  tin  are  found  to  be  very 
difficult  to  collect.  In  order  that  the  fusion  may  be  successful, 
it  is  essential  to  follow  directions  closely.  It  is  best  to  use  10 
grams  of  concentrates,  or  an  amount  near  that;  usually  the 
amount  of  concentrates  obtained  from  the  concentration  of  the 
ore  approximates  this  if  the  proper  amount  of  ore  is  chosen  for 
concentration.  Two  grams  of  powdered  cyanide  are  firmly 
tamped  into  a  20-gram  crucible,  the  concentrates  are  mixed 
with  30  grams  more  of  cyanide,  placed  in  the  crucible,  and 
covered  with  5  grams  more.  The  crucibles  are  placed  in  the 
muffle  at  a  full-red  heat  (750°  C.),  and  are  kept  at  this  temper- 
ature for  about  15  to  20  minutes.  The  charge  will  become  very 
liquid,  and  will  be  a  brown-red.  The  temperature  should  not 
be  so  high  as  to  cause  the  cyanide  to  boil  and  evolve  heavy 
fumes.  It  may,  however,  be  kept  too  low,  in  which  case  the 
chemical  reactions  will  not  complete  themselves  and  the  tin 
will  fail  to  collect  into  a  button.  If  the  concentrates  still  con- 
tain some  foreign  minerals,  the  fusion  takes  longer  than  20 
minutes.  The  crucibles  are  then  withdrawn,  cooled,  and  the 
button  recovered  by  breaking  the  crucible.  There  will  be  two 
distinct  slags,  the  lower  one,  surrounding  the  button,  usually 
light  green,  amorphous  and  subtranslucent,  and  the  upper  one, 
or  fused  cyanide,  opaque,  milk-white  and  coarsely  granular, 
soluble  in  water.  The  tin  button  should  be  white  and  soft;  if 
not,  it  contains  foreign  metals. 

The  German  Method. — The  German  method  is  based  on  the 

i  Hofman,  ibid. 


196  *        A   MANUAL    OF    FIRE    ASSAYING 

fusion  of  the  cassiterite  concentrates  with  charcoal  and  black 
flux  substitute,  which  has  the  composition,  2  parts  K2CO3,  1  part 
flour.  Five  grams  of  the  concentrates  are  intimately  mixed  with 
1  gram  of  pure  wood  charcoal  and  put  into  a  No.  D  lead  crucible  or 
an  ordinary  20-gram  crucible.  On  top  of  this  are  placed  15  grams 
of  black  flux  substitute,  with  which  1.25  grams  borax  glass  have 
been  mixed.  Finally  a  pure  salt  cover  is  added,  and  a  piece  of 
charcoal,  the  crucible  covered  with  a  clay  cover,  placed  in  the 
muffle,  and  heated  at  a  moderate  heat  until  boiling  of  the  charge 
has  ceased,  and  then  for  one-half  to  three-quarters  of  an  hour 
more  at  a  white  heat.  The  crucible  is  then  removed  from  the 
muffle,  allowed  to  cool,  and  broken  for  the  tin  button.  This 
should  be  white  and  soft,  as  in  the  cyanide  fusion. 

During  the  fusion,  as  the  temperature  rises,  the  charcoal 
reduces  the  stannic  oxide  to  metallic  tin,  while  any  ferric  oxide 
is  reduced  to  ferrous  oxide,  if  the  heating  is  gradual,  and  is  taken 
up  by  the  slag.  As  the  temperature  rises,  the  flour  in  the  black 
flux  substitute  partially  decomposes,  liberating  carbon  through- 
out the  charge,  which,  as  fusion  takes  place,  prevents  any  stannic 
oxide  not  as  yet  reduced  from  uniting  with  the  alkali  of  the 
flux.  The  slag,  after  cooling,  should  be  crushed  and  panned  for 
any  prills  of  tin  which  have  not  entered  the  button.  These  are 
weighed  and  added  to  the  weight  of  the  button. 

Results  Obtainable. — Black  Hills  cassiterite  concentrates, 
roasted,  quenched,  and  treated  with  nitro-hydrochloric  acid.1 

Wet    method    of    Rose-Chauvenet 

with  K2CO3 =67.84  per  cent.  Sn 

German  method =  67 . 58  per  cent.  Sn 

Cyanide  method =67.49  per  cent.  Sn 

Stream  tin  from  Durango,  Mexico,2 

Wet  method  (Rose) =65.62  per  cent.  Sn 

German  method =63 . 92  per  cent.  Sn 

Cyanide  method =  65 . 19  per  cent.  Sn 

It  is  to  be  noted  that  while  the  dry  methods  approach  very 
closely  to  the  wet  analysis,  which  gives  the  actual  tin  in  the  ore, 
the  dry  assay  results  are  due  more  or  less  to  a  balancing  of  errors. 
Frequently  dry  assays  will  give  higher  results  than  the  analysis; 
this  is  due  usually  to  reduced  iron. 

1  Hofman,  ibid. 

2  E.  H.  Miller.  "The  Assay  of  Tin  Ores,"  in  "School  of  Mines  Quart.,"  XIII,  No.  4. 


THE    ASSAY    OF    TIN,    MERCURY,    LEAD,    ETC.  197 

Of  the  influence  of  foreign  minerals  left  in  the  cassiterite 
concentrates,  quartz  has  the  worst,  causing  heavy  losses.  Feld- 
spar and  tourmaline  have  similar  effect,  but  not  to  so  marked  a 
degree.  Mica  and  garnet  give  high  results,  due  to  the  reduction 
of  iron,  although  tin  is  lost  in  the  slag.  Columbite  acts  in  a 
similar  manner.  With  the  German  method  the  result  is  much 
more  seriously  affected  by  these  impurities  than  with  the  cyanide 
fusion.1 

THE  ASSAY  OF  MERCURY.— Mercury  occurs  in  ores  chiefly  as 
cinnabar  (HgS),  and  may  with  accuracy  be  determined  by 
Chism's  method.2  For  low-grade  ores,  the  method  is  especially 
satisfactory,  and  has  the  advantage  of  being  rapid  and  short.  It 
is  based  on  the  fact  that  mercury  is  distilled  from  HgS,  etc.,  in 
the  presence  of  iron  filings,  and  can  be  caught  on  silver-foil.  The 
difference  in  weight  between  the  mercury-impregnated  silver- 
foil  and  the  foil  before  the  assay  gives  the  mercury.  The  appa- 
ratus required  is  as  follows: 

1.  A  small  ring-stand. 

2.  A  fire-clay  annealing  cup  (No.  B  or  C). 

3.  A  piece  of .  carefully   annealed   silver-foil    1.5   in.   square, 
which  is  fitted  and  bent  down  to  make  a  reasonably  tight  cover 
for  the  annealing  cup. 

4.  A  flat  silver  or  copper  dish,  holding  20  to  25  c.c.  of  water. 
A  silver  crucible  may  be  used  in  place  of  this. 

5.  A  piece  of  asbestos  board,  4  in.  square  and  about  0.20  in. 
thick,  in  the  center  of  which  a  circular  hole  has  been  carefully 
cut,  into  which  the  annealing  cup  will  fit  so  as  to  project  about 
0.5  in.  below  the  bottom  of  the  board. 

6.  A  small  alcohol  lamp,  of  about  60  c.c.  capacity. 

7.  A  wash-bottle  with   cold  water,   and  a  glass  tube  for  a 
siphon.     The  silver-foil  is  carefully  fitted  over  the  top  of  the 
annealing  cup,  the  edges  being  bent  down  so  as  to  make  a  close- 
fitting  cover  and  prevent  the  escape  of  mercurial  vapor.     The 
silver  dish  should  be  polished  on  the  bottom,  and  be  in  close 
contact  with  the  foil,  so  that  the  cooling  effect  of  the  water  will 
be  fully  transmitted. 

The  Assay. — For  low-grade  ores  from  0.5  to  1  gram  is  taken 
and  mixed  with  from  30  to  50  parts  of  iron  filings.  These  filings 

1  Hofman,  ibid. 

2  R.  E.  Chism,  in  Trans.  A.  I.  M.  E.,  XXVIII,  444. 

Consult  also,  G.  A.  James,  Eng.  and  Min.  Jour.,  XC,  800  and  W.  W.  Whitton,  Calif. 
Tech.  Jour.,  Sept.,  1904;  Min.  Ind.,  XVII,  751. 


198  A    MANUAL    OF    FIRE    ASSAYING 

should  all  pass  a  40-mesh  screen.  A  select  lot  of  filings  are  best 
digested  with  alcohol  for  some  time  to  remove  oil  and  grease, 
then  heated  in  a  muffle  to  a  dull-red  heat  for  10  minutes,  cooled, 
and  stored  in  a  tight  bottle.  It  is  essential  to  have  the  filings 
free  from  oil  and  grease,  else  this  will  be  deposited  on  the  silver- 
foil  with  the  mercury.  The  amount  of  mercury  in  the  ore  should 
not  be  so  great  as  to  cause  too  heavy  a  coat  on  the  silver-foil. 
For  high-grade  ores,  not  more  than  0.1  to  0.2  gram  should  be 
used.  Very  small  amounts  of  mercury  can  be  detected  by  this 
method. 

The  ore,  mixed  with  filings,  is  placed  in  the  annealing  cup, 
which  is  set  into  the  asbestos  board  on  the  ring-stand,  the  silver- 
foil  weighed  accurately,  after  igniting,  to  within  0.1  mg.,  and 
fitted  to  the  cup,  and  the  silver  dish,  filled  with  cold  water,  placed 
on  the  foil.  The  alcohol  flame  is  then  allowed  to  play  just  on 
the  bottom  of  the  cup,  but  not  to  spread  around  the  sides.  The 
flame  should  be  about  1.25  in.  high  and  is  best  shielded  by  a 
screen  to  steady  it.  The  bottom  of  the  crucible  should  not 
become  more  than  a  dull  red,  otherwise  mercury  will  escape 
condensation.  The  time  of  heating  should  be  from  10  to  15 
minutes.  It  is  best  to  heat  for  about  10  minutes,  then  cool, 
and  reheat  for  3  to  5  minutes.  Longer  heating  than  this  causes 
loss  of  mercury.  The  degree  and  time  of  heat  are  very  important. 

During  the  heating  the  water  in  the  dish  should  be  replaced 
once  or  twice.  It  can  easily  be  removed  by  a  bent  tube  that 
has  been  filled  with  water,  acting  as  a  siphon.  While  the  warm 
water  is  being  removed,  cold  water  is  added  from  a  wash-bottle. 
After  the  proper  heating,  the  alcohol  lamp  is  removed,  the  assay 
allowed  to  cool  somewhat,  the  silver  dish  removed,  and  the 
silver-foil  with  the  mercury  transferred  by  forceps  to  a  desiccator 
and  then  weighed.  The  difference  in  the  weight  of  the  foil  after 
and  before  the  assay  is  the  weight  of  the  mercury,  from  which 
the  percentage  is  calculated.  The  foil  can  be  used  again  after 
driving  off  the  Hg  at  a  red  heat  in  the  muffle,  or  with  a  Bunsen 
burner.  A  piece  of  foil  can  be  used  about  six  times.  It  should 
be  weighed  before  each  assay.  The  method  also  serves  as  a  very 
sensitive  and  easily  applied  qualitative  test  on  ores. 

The  following  figures  will  serve  to  show  the  accuracy  of  the 
method  :* 

1 G.  N.  Bachelder,  in  "School  of  Mines  Quart.,"  XXIII,  98. 


THE    ASSAY    OF    TIN,    MERCURY,    LEAD;    ETC. 


199 


BY  ELECTROLYSIS  FROM  CYANIDE 

SOLUTION 

Ore  No.  1  12.37  per  cent. 

Ore  No.  2  67.26  per  cent. 


BY  CHISM'S  METHOD 

12.44  per  cent. 
67 . 23  per  cent. 


The  accompanying  illustration  (Fig.  60)  shows  the  apparatus 
employed. 

THE  ASSAY  OF  LEAD  ORES.— The  fire  assay  of  lead  ores  will 
probably  pass  out  of  use  in  time,  just  as  the  fire  assay  of  copper 
has  done.  At  the  present  time  it  is  still  largely  used,  although 
for  complex  ores  containing  much  copper  or  bismuth,  or  antimony 
with  the  lead,  it  is  not  in  vogue.  It  is,  however,  still  the  criterion 


FIG.  60. — APPARATUS  REQUIRED  FOR  THE  MERCURY  ASSAY. 

in  the  purchase  of  pure  sulphide  and  oxidized  lead  ores,  and  also 
such  complex  ores  as  furnished  by  the  Leadville,  Colorado, 
district.  Unoxidized  ores  of  this  type  contain  pyrite,  blende, 
galena,  some  little  chalcopyrite  and  gangue.  Oxidized  ores 
contain  cerrusite,  anglesite,  calamine,  limonite,  etc.,  and  gangue. 
The  object  of  the  assay  is  to  bring  the  lead  of  these  ores  down 
into  a  button,  free  from  other  base  metals,  such  as  Cu,  Zn,  Bi,. 
Sb,  Fe,  and  free  also  from  S  and  As.  The  loss  of  lead  by  volatili- 
zation and  slagging  and  the  reduction  of  base  metals  should  be 
kept  to  a  minimum.  As  already  stated,  this  is  a  difficult  thing 


200  A   MAXUAL    OF    FIRE    ASSAYING 

to  do;  so  that  pure  ores  will  invariably  give  low  results,  and 
impure  ones  high. 

There  are  three  methods  of  assay,  differing  in  the  flux  used; 
(1)  the  lead  flux  method;  (2)  the  soda-argol  method;  (3)  the 
cyanide  fusion.  Of  these,  the  lead  flux  method  is  chiefly  used 
throughout  the  West.  The  soda-argol  method  is  a  good  one  on 
ores  not  basic.  The  cyanide  method  is  only  applicable  to  pure 
ores.  With  impure  ores  it  tends  to  reduce  other  base  metals, 
due  to  its  powerful  reducing  action.  Various  mixtures  of  lead 
flux  are  used,  of  which  three  are  made  up  as  follows: 

No.  1  No.  2  No.  3 

4  parts  NaHCOs  2  parts  NaHCO3  6.5  parts  NaHCO3 

4  parts  K2CO3  2  parts  K2CO3  5      parts  K2CO3 

2  parts  flour  1  part  flour  2  .  5  parts  flour 

1  part  borax  glass  1  part  borax  glass  2  .  5  parts  borax  glass 

Flux  No.  3  is  probably  the  best  for  most  purposes,  as  deter- 
mined on  a  series  of  ores,  the  results  with  it  being  slightly  higher.1 
For  assay,  10  grams  of  ore  (100-mesh  fine)  are  mixed  with  30 
grams  of  flux,  placed  in  a  No.  6  or  D  crucible,  or  in  a  20-gram 
crucible,  covered  with  8  grams  more  of  flux,  and  put  into  the 
muffle  at  a  low  heat,  \vhich  is  then  raised  to  a  light  yellow  (1080° 
C.).  The  fusion  should  take  about  30  to  35  minutes.  Nails 
are  added  to  the  charge.,  two  tenpenny  nails  for  heavy  sulphides, 
one  for  light  sulphides  or  oxidized  ores.  When  the  charge  is 
taken  from  the  muffle,  the  nails  are  removed  from  the  crucible 
by  a  pair  of  short  hand  tongs,  care  being  taken  to  wash  off  all 
adhering  lead  globules.  The  crucible  is  then  shaken  and  tapped 
thoroughly,  and  poured.  The  lead  buttons  are  cleaned  by 
hammering  and  then  weighed.  The  percentage  is  obtained  by 
multiplying  by  10. 

The  reactions  in  the  crucible  are  as  follows  : 
7PbS  +  4K2C03  -  4Pb  +  3(K2S,  PbS)  +  K2SO4  +  4CO2 
K,S,  PbS  +  Fe  =  Pb  +  K2S  +  FeS 


The  carbon  liberated  in  finely  divided  particles  from  the 
flour  on  heating  reduces  any  lead  oxides  or  carbonates  in  the  ore, 
while  the  iron  reduces  lead  from  its  sulphides  and  sulphates. 
The  assay  should  check  (in  triplicate)  within  0.5  per  cent. 

>McElvenny  and  Izett,  in  "The  Chemical  and  Fire  Methods  of  Determining  Lead  Ores," 
Min  Rep.,  XL  VIII,  26. 


THE   ASSAY    OF    TIN,    MERCURY,    LEAD,    ETC.  201 

The  soda-argol  method  uses  the  following  flux: 

NaHCO3 6  parts 

Argol 1  part 

For  10  grams  of  ore,  35  grams  of  flux  are  taken,  with  a  light 
flux  cover.  The  fusion  is  performed  as  described  for  the  lead 
flux  method.  The  method  is  good  on  ores  containing  some 
silica,  but  not  on  basic  ores  or  pure  galenas,  as  all  acid  is  lacking 
in  the  flux.  A  borax  glass  cover  is  best  where  the  method  is 
employed  on  basic  ores. 

In  the  cyanide  method,  pure  cyanide  should  be  used,  and  the 
temperature  should  be  kept  much  lower  than  for  the  other  two 
methods.  For  the  regulation  of  temperature,  reference  is  made 
to  the  assay  of  tin  by  the  cyanide  fusion. 

For  the  fusion,  10  grams  of  ore  are  mixed  with  35  grams 
cyanide,  and  a  light  cyanide  cover  used.  Concerning  the  accuracy 
of  the  method  the  following  figures  are  appended:1 


Fire  assay  (lead  flux)    Gravimetric  (PbSOJ 
Per  cent.  Per  cent. 


1.  Galena 

76 

78.68 

2.  Galena  
3.  Cerrusite  

37 
9 

37.40 
10.60 

4.  Pyrite,  Sphalerite,  Galena 
5.  Galena  and  Stibnite  
6.  Cerrusite  

24.7 
28.7 
37.8 

18.46 
27.25 
38.60 

THE  ASSAY  OF  ANTIMONY  AND  BISMUTH  ORES.— For  accurate 
and  satisfactory  determinations  on  these  ores,  wet  methods  must 
be  resorted  to.  Antimony  occurs  chiefly  as  the  sulphide,  stibnite, 
although  the  oxides  and  some  native  metal  are  found  as  ore. 
Bismuth  as  an  ore  occurs  chiefly  as  the  native  metal,  but  is  found 
also  in  combination  with  oxygen,  sulphur,  etc.  For  the  assay, 
the  following  charge  is  best: 

Ore 10  grams 

Cyanide 40  to  50  grams 

Cover  of  cyanide. 

Fuse  at  a  full  red  heat,  as  given  for  tin,  for  30  minutes.  The 
resultant  buttons  are  brittle  and  cannot  be  hammered. 

1  Determination  of  Lead  in  Ores,  I.  T.  Bull,  School  of  Mines  Quart.,  XXII,  348. 


APPENDIX 


1 

sit       I 
HI 

&  i 
&    I   s    s 

3 

(N           0)          US                         -* 
rt           N          00           £             • 

N°          "*                         S?          I- 

§       ^       c^       § 

1    ! 

E         °° 

S 

1 

O           CO          h-          M<          •** 
CO           O          <O          O          00 

4 

|     a 

"J<          JH          0          00          00 

I           §     I 

*     1     S     9 

0 

§0 

8 

3 

0 

«    fe    N 

3                 § 

'§.  » 

i      !5      ^ 

1     « 

II 

•<j*       ci       t>-       »o       o 

o       o 

•2  | 

|     |     1     ?!     1 

111 

>• 

< 

. 

^                          c-i 

|, 

1    S    §    s 

1     8 

11 

iH             CO            rt 

S     l      '     1 

§ 

CO                                                     ^ 

I 

1 

11         •       1 

i   I   1    10 

I 

-  2   • 

s.  ;  •  S 

f 

1           1 

«          8          CO 

sis 

1 

•*  a  i  a 

-l                                       CO 

?,                  S 

S 

S          <N 

i 

•3 

§ 

S     3 

o 

^           W           00          S          M 

i        ^    i 

0 

2 

;                  ;         :        o 

1    j     ; 

•i  !  I  H 

0       (2       S       H       «< 

till 

•<    a    c    w 

202 


APPENDIX 


203 


3     S     3 


S     5 

g 


Q          !N 


IN          IN          IN 


<0  S  O 

§82 


J_l 


00          •*          t~ 


8   -8-  - 


g 


L 


I  I  I  i 


-S8 

H 


s  i 


it!'] 

•s    I    i    -s 


&    i 


s  s  §  I 

ft  0-  K  £ 

g  2  a  8 

§  a  £  s 

°  §  £  « 

Sill 

§  s  §  § 


204  A    MANUAL    OF    FIRE    ASSAYING 

TABLE  XLIII.— FINENESS  OF  BULLION  AND  ALLOYS  OF 
PRECIOUS  METALS 


Equivalent  in 

' 

Denomination 

Milliemes  or  Parts 

Per  Thousand 

One  carat  

41.666 

/  24  carats  = 
1  24  carats  = 

1  pound  troy  (England) 
1  mark  (Germany,  etc.) 

One  grain  per  marc.  .  .  . 

.217 

j  4,608  grains 
\    Spain,  etc.) 

=  1  marc  ol  8  ounces  (France, 

One  ounce  per  marc.  .  . 

125.000 

8  ounces  = 

1  marc  (France,  Spain,  etc.) 

One  loth  (silver)  

62.500 

16  loth  =  1 

mark  (Germany,  etc.) 

TABLE  XLIV.— VOLUME  AND  WEIGHT  OF  FINE  GOLD  AND 
SILVER 


One  Cubic 
Centimeter 

One  Cubic 
Inch 

One   Cubic 
Foot 

Fine  Silver: 
Weight:  grams  
Weight:  troy  ounces  

10.57 
.339825 

173.21 
5.5687 

299307.00 
9622.72 

Fine  Gold: 
Weight:  grams  
Weight:  troy  ounces  
Value:  U.  S.  dollars  -.  

19.3 
.6205 

$12:82" 
£2.647 

316.269 
10.1680 
$210.17 
£43.214         H 

546,513 
17,570.39 
$363,180 
£74,674 

The  foregoing  tables  are  due  to  Mr.  W.  J.  Sharwood  and  were 
first  published  in  Mines  and  Minerals,  XXIX,  250. 

Bases  of  Computation. — The  gram  is  taken  as  15.4320  grains. 
The  value  of  a  troy  ounce  of  fine  gold  is  assumed  as  being  exactly 
$20.67,  instead  of  $20.6718346+,  resulting  in  an  error  of  less 
than  one  in  10,000.  Values  in  English  coin  are  based  on  the 
assumption  that  an  ounce  of  fine  gold  is  worth  4.25  pounds  ster- 
ling, or  85  shillings,  1,020  pence;  this  is  too  high  by  about  one 
part  in  2,000,  the  true  value  being  1,019.45  pence.  It  is  useless 
to  attempt  a  closer  approximation  in  practical  work,  for  the 
simple  reason  that  gold  bullion  assays  are  rarely  reported  closer 
than  the  nearest  half  millieme,  or  to  within  one  part  in  2,000. 
At  the  values  adopted  one  dollar  is  equivalent  to  4.11224  shillings, 
and  one  pound  sterling  to  $4.86353. 

Foreign  and  Obsolete  Values. — The  adarme  (27.7  grains  or  T*g- 
of  the  Spanish  ounce),  sometimes  used  by  Mexicans  colloquially 


APPENDIX  205 

and  especially  with  reference  to  placer  work,  is  about  1.8  grams, 
which  in  fine  gold  would  be  worth  $1.20.  For  practical  purposes, 
however,  an  adarme  of  ordinary  gold  may  be  taken  as  equivalent 
to  $1,  and  this  exactly  true  for  gold  830  fine. 

Russian  reports  state  values  in  zolotniks  per  100  poods,  but  for 
low-grade  placer  deposits  doli  per  100  poods  are  used.  As  a  dola 
is  gV  zolotnik,  we  may  take  yf^  of  the  values  given  in  the  table 
for  the  zolotnik,  without  serious  error. 

Marcos  per  cajon  were  formerly  used  in  some  South  American 
countries,  one  marco  per  cajon  apparently  ranging  between  100 
and  70  parts  per  million.  One  oitavo  per  quintal  corresponds 
practically  to  2  ounces  per  ton. 

The  loth  per  centner,  used  in  the  older  German  works,  corre- 
sponds to  one  part  in  3,200,  which  is  nearly  9  ounces  per  short  ton 
or  10  ounces  per  long  ton.  In  some  cases  there  seems  to  have 
been  a  considerable  variation  from  this  ratio,  the  value  being 
sometimes  taken  as  one  part  in  3,520,  or  yy  of  that  used  in  these 
tables — the  centner  being  then  assumed  as  110  instead  of  100 
pounds.  The  quentchen  was  \,  and  the  denar  -^  of  the  loth. 


INDEX  TO  AUTHORS 


NAME  PAGE 

Allen,  E.  T.,  see  Hillebrand  and  Allen. 

Ames  and  Bliss 48 

Anderson,  O.  A 104 

Austin  and  Hunter 48 

Bachelder,  G.  N 198 

Bailar,  J.  C 101 

Balling,  C 69 

Bannister  and  Stanley -. 103 

Barton,  W.  H      157 

Bettel,  W I....'.....'....'..     77 

Bowman,  F.  C 3 

Bowman  and  Mason 160 

Brunton,  W.  D 36 

Bull,  I.  T      . 201 

Bullens,  D.  K ,...../ 147 

Carter,  T.  L. 145 

Chiddey,  A 157 

Chism,  R.  E 197 

Clark,  A.  J 156 

Clennell,  J.  E 30,  180 

Collins 94 

Crawford,  C.  H 129,  168 

Day  and  Allen 30 

Day  and  Shepard 29 

Dewey,  F.  P. 172 

Doeltz,  O , 28,  90 

Doeltz  and  Graumann 64,  90 

Donnan  and  Shaw 83 

Eager  and  Welch 162,  164 

Edmands,  H.  K 79 

Flinn,  F.  B 127 

Friedrich,  K 84 

Fulton,  C.  H 86,  104,  129,  134,  137,  160,  167,  168,  169 

Godshall,  L.  D 161,  162 

Gottschalk,  V.  H     .    .    .    .    : .- 42 

Guertler,  W      29 

Hall,  E.  T 149 

Hawley,  F.  G 108,  149 

Hempel,  A 31,  77 

Hemtz,  F 76 

Hillebrand,  W.  F.,  and  Allen       .  128,  132,  137,  164,  165,  166,  167,  171,  172 

Hofman,  H.  O 117,  194,  195,  196,  197 

Holloway  and  Pearse      133 

207 


208  INDEX   TO    AUTHORS 

NAME  PAGE 

Holt  and  Christensen 79,  106 

Howe,  H.  M 94,  102 

Hunt,  F.  F 126 

Huntoon,  L.  D 40 

Izett  and  McElvenny      ...*.... 200 

James,  G.  A 39,  197 

Janin,  Jr.,  Louis      6 

Jolly,  H.  R 159 

Kaufman,  W.  H 160,  162 

Keller,  Edward 19,  41,  107 

Kemp,  J.  F 186 

Kerl,  B 76 

Kitto,  Wm 146 

Koenig,  G.  A 1 

Lay,  F 144 

Lenher,  Viet 172 

Liddell,  D.  M 41,  47,  105,  162 

Lodge,  R.  W 118,  119,  147,  161,  164,  167,  168,  188 

Mason  and  Bowman 160 

McCaughey,  W.  J 129 

Merritt,  J.  W 79,  106 

Miller,  E.  H.    .    .    .    54,  59,  116,  120,  160,  168,  169,  186,  187,  188,  191,  196 

Mostowitsch,  Wl 28,  64,  65,  90,  91 

Nutter,  E.  H 6 

Orton,  E 24 

Ostwald,  W 48 

Pack,  J.  W .  181,  184,  185 

Perkins,  W.  G 116,  140,  142 

Roberts,  G.  M 41 

Roberts-Austen       29,  93 

Roos,  A.  T 153 

Rose,  J.  G 33 

Rose,  T.  K 94,  99,  107,  124,  134,  143,  144,  164,  165,  171,  172,  181, 

182,  183,  184,  185,  187,  188 

Sander,  K 99,  146 

Schnabel,  C 94 

Schiffner,  M 187 

Sharwood,  W.  J 154,  187,  188,  202 

Shepard  and  Day 29 

Schorlemmer  and  Roscoe 64 

Sieverts  and  Hagenacker 83 

Smith,  E 99 

Smith,  E.  A 146 

Smith,  F.  C 101,  137 

Smith,  S.  W 133,  134,  136 

Sulman,  H.  L 146 

Taylor,  E.  H.   . 180 

Thompson,  T 186,  187 


INDEX   TO    AUTHORS  209 

NAME  PAQB 

Vail,  W.  G 149 

Van  Liew,  R.  W 126 

Van  Nuys,  C.  C 164 

Vogt,  J.  H.  L 67 

Wallace,  R.  C 67 

Warwick,  A.  W 37 

West,  E.  E 68 

Whitby,  A 157 

White,  W.  P 33,  54 

White  and  Taylor 102 

Whitton,  W.  W 197 

Williams,  J.  D 143 

Williams,  K 117 

Woodward,  E.  C 121,  131,  167 

Wraith,  W 41 

Wright,  L.  T 37 


INDEX 
A 

PAOB 

Accuracy  of  the  gold-silver  assay 173 

Alternate  shovel  method  of  sampling 38 

Alumina 31 

Amalgamation  Test 154 

Analysis  of  bone  ash 76 

of  copper  matte 142 

of  hematite 74 

of  fine  silver 171 

of  fire  clay  for  crucibles 25 

of  lead-antimonial  ores 73 

of  limestone      74 

of  sheep  and  cattle  bones 76 

of  silicious  ores „ 73 

Antimony,  behavior  of,  in  roasting 114 

ores,  assay  of 201 

Antimonial  ores,  method  of  assay  of 146 

Argol 31 

reducing  power  of 53 

Arsenic,  behavior  of,  in  roasting       114 

Arsenical  ores,  method  of  assay  of 146 

nickel-cobalt  silver-ores      147 

method  of  assay  of 147 

Assay  balance 48 

reagents 28 

valuations,  table  of .  203 

Assaying,  definition  of 27 

Assay-ton  system  of  weights 51 

B 

Balances 42 

construction  of 42 

for  weighing  pulp 49 

non-column  type  of 49 

practical  notes  on 47 

theory  of 44 

Basic  ores 62 

Bismuth  ores,  assay  of 201 

Black  flux 32 

substitute 32 

211 


212  INDEX 

PAGE 

Blister  Copper,  method  of  assay  of      125,  126 

sampling  of 41 

Bone-ash,  analysis  of 76 

screen-analysis  of 77 

Borates 30 

Borax 29 

glass 29 

influence  of  on  assay 30,  138 

melting  point  of 30 

Boric  acid 29 

Bullion,  assay  of 174 

classification  of 174 

fineness  of 174,204 

Burners,  Case  type 15 

consumption  of  oil  by 15 

for  gasolene 13 

size  of 14 

for  oil 4 

for  gas 17 

pressure  of  gasolene  in 14, 15, 17 

C 

.Gary  burner 14 

Case  gasolene  burner 15 

Charcoal 32 

reducing  power  of 54 

temperature  at  which  reaction  begins  with 59 

use  of,  in  roasting   . 114 

Check  assay 176,184 

Chism's  method  for  mercury 197 

Coning  and  quartering,  sampling  by 37 

Control  assays 39 

Combination  method  of  assay 125,  127 

for  cyanide  precipitates 129,  144 

precautions  to  be  observed  in 128 

Copper  mattes,  method  of  assay  of 127 

results  on      . 141 

method  of  assay  of  blister 125,  126 

sampling  of  blister 41 

Copper-bearing  material,  assay  of 125,  127,  139,  140,  142,  151 

losses  in  assay 169 

Cost  of  fuel  for  assaying 3,  5,  6,  8,  15 

Crucibles 24 

Battersea      25 

capacity  of 25 

domestic 25 

fire-clay  for * 25 


INDEX  213 

PAGE 

Crucible  assay,  methods 113 

theory  of 63 

charges  for  quartz  ores 74 

for  basic  ores 70 

Cupels,  assay  of 140,  158,  175 

comparison  of  bone  ash  and  magnesia 104,  105 

cost  of,       79 

influence  of  shape  of 78 

of  bone  ash 76,  103 

of  magnesia    . 79,  103,  104 

of  Portland  Cement 79,  105,  106 

properties  of 77,  78,  103 

Cupel-charging  device 21 

Cupel  machines 77 

Cupellation,  appearance  of  beads  from 84 

when  platinum  is  present 187 

"flash"  of  beads  after 94 

formation  of  "feathers"  during 82,93 

"  freezing  "  of  lead-button  in 88,92 

influence  of  copper  on 100,  101 

of  antimony  on 102 

of  foreign  metals  on 81,  92,  98 

of  tellurium  on 101,  102,  133 

losses  in 160 

nature  of 76 

process  of 80 

retention  of  lead  in  beads  from 171 

silver  and  gold  absorption  during 78,  104,  105,  106 

sprouting  of  silver  after 83 

surfusion  of  silver  during 82,  93,  94,  97 

temperature  of 81,  86,93 

uncovering  of  lead-button  in 88 

Cyanide 32 

of  "Potassium" 32 

of  sodium      32 

Cyanide  method  for  antimony  ores      201 

for  bismuth  ores 201 

for  gold  and  silver  ores 120 

results  by 120 

lead  ores 201 

tin  ores 195 

Cyanide  Solutions,  assay  of 156,  157 

D 
Dore"  bullion 174 

E 
Errors  in  the  assay  for  gold  and  silver 173 


214  INDEX 

PAGE 

Excess  litharge  method      116,142 

results  obtainable  from 117,  118 

F 

Ferric  oxide 31 

Flour 32 

reducing  power  of 54 

Fluor  spar 31 

in  the  reassay  of  cupels      158 

Furnaces,  capacity  of   .    . 2,  5,  6,  15 

combination  pot  and  muffle      10 

for  assaying      1 

for  burning  coal 2 

coke 6 

gas 17,18 

oil 4,  17 

wood      5, 6 

fuel  consumption  in <. 3,  5 

gasolene-fired 12 

temperature  attainable  in      15 

Furnace  tongs      18, 19 

tools 18 

annealing-cup  tray      23 

cupel-charging  device 21 

cupel-tray 23 

molds 23 

.    multiple  scorifier  tongs 19 

G 

Gasolene  as  fuel  in  assaying      12, 15 

German  method  of  assay  for  tin 195 

Gold,  losses  of  during  cupellation 164 

occluded  gases 172 

preparation  of  proof 185 

silver  retained  by  after  parting 171 

solution  of  by  acid      172 

weight  and  volume  of  fine 204 

Gold  bullion 174 

assay  of 181 

for  silver  in 180 

surcharge  in  assay  of      173,  185 

ores  containing  "free"  gold,  assay  of      152,  154 

-silver  alloys,  losses  during  cupellation  of 164 

Graphitic  material,  method  of  assay  for 145,  146 

H 
Hematite,  effect  of  in  ores 62 


INDEX  215 


PAGE 

Impurity,  definition  of Ill 

Impurities  in  ores Ill,  112 

Inquartation 108 

Iridium,  determination  of .    .    .    .    191,  192 

effect  of  acids  on 188 

in  platinum  nuggets 186 

Iron,  assay  of  metallic 159 

nail  method 118 

reactions  in .119 


Jones  riffle  sampler 39,  40 


Lead 31 

bullion,  assay  of      175 

sampling  of 41 

flux      32,200 

ores,  assay  of 199 

fluxes  used  in  assay  of 200 

reactions  dunng  assay  of 200 

results  obtained  in  assay  of 201 

silicates 65 

formation  of  in  roasting 115 

melting-point  of 91 

reduction  of 55 

Lime 31 

Litharge 28 

melting-point  of 28,  90 

reduction  of 53,  55,  64,  135 

required  to  fuse  metallic  oxides 117 

silver  and  gold  in 34,  35 

Luting  material 9,  11 

M 

Matte  produced  in  crucible  assay Ill,  112 

Mercury,  assay  of 197 

results  obtained  in       199 

Method  of  assay  for  antimony      201 

for  bismuth       201 

for  copper  bearing  material 125,  127,  139,  151 

for  cyanide  solutions 156,  157 

for  free  gold  ores 152,  154 

for  lead 199 

for  mercury      197 


216  INDEX 

PAGE 

for  platinum  ores 189 

for  telluride  ores 131 

for  tin 193 

Methods  of  assay 113 

combination  method 125,  127 

comparison  of       121 

cyanide  method 120 

iron  nail 118 

Miller's  oxide  slag  method 116 

niter  method •. 115 

niter-iron  method 120 

Perkins'  excess  litharge  method 116 

roasting  method      113 

scorification  method 122 

Miller's  oxide  slag  method 116 

Molds 23 

Muffles,  method  of  support  of       8 

size  of    .  8 


N 

Niter 33 

iron  method  of  assay  120 

manner  of  action  of 59 

method  of  assay  115 

oxidizing  power  of 57,  58,  60 

against  various  reducing  agents 59 

variation  of  oxidizing  power  of 60 

Nitric  acid,  action  of  on  platinum  alloys 187 

used  in  parting 107 

Nitro-hydrochloric  acid,  action  of  on  platinum  alloys      188 

O 

Oxidation 57 

metals,  sequence  of 124 

Oxidizing  power  of  an  ore 62 

P 

Palladium,  behavior  of  during  assay 190 

effect  of  acids  on 187 

Parting  107 

precautions  during  108 

ratio  of  gold  to  silver  necessary 107,  108,  181 

strength  of  acids  used  in , 107 


INDEX  217 

PAGE 

Platinum  alloys,  behavior  of  with  acids, 187 

and  allied  metals,  behavior  of  during  assay 186 

ores,  assay  of 186 

methods  of  assay  of 189 

Potassium  carbonate 29 

cyanide 32 

nitrate 33 

Preliminary  assay 61 

Pyrite,  reducing  power  of 54,  55,  61 

R 

Reagents  for  assaying 28 

assay  of 33 

Reducing  agents 55 

amount  of  lead  reduced  by 55,  56 

power  of  an  ore 61 

Reduction 53 

Roasting 113 

dishes 26 

S 

Salt 33 

Sampling  .    . ' 36 

by  alternate  shovel  methods 38 

by  coning  and  quartering      37 

by  machine 37 

of  copper  and  lead  bullion 41 

of  gold  and  silver  bullion 181 

salting  during 40 

Scale  ores,  methods  of  assay  of 151 

Scorification 122 

application  of  method 125 

errors  in  method      170 

for  copper  bearing  material 139,  141 

for  zinc  ores .' 144 

process  of 123,  124 

Scorifiers 24 

capacity  of 26,  122 

Selenium  gold  ores      131 

Silica 29 

Silver,  behavior  of  in  roasting 1 14 

bullion 174 

cupellation  method  for 175 

Gay-Lussac  method  for      177 

gold  alloys,  losses  during  cupellation  of      164 

losses  of  during  cupellation 160 

method  of  adding  to  assay      35,  108 


218  INDEX 

PAGE 

method  for  in  gold  bullion 180 

preparation  of  proof 185 

retained  by  parted  gold 171 

weight  and  volume  of 204 

Slags,  assay      65 

calculation  of 69 

color  of 75 

formation  temperature  of 67,  68 

nature  of 67 

assay  of 158 

silicate  degree  of 66 

Soda-argol  method  for  lead  ores 201 

Sodium  bicarbonate 29 

carbonate 28 

influence  of  on  reduction  of  litharge 54 

melting-point  of 29 

chloride 33 

cyanide 32 

sulphate,  formation  of  in  assay 54 

Sprouting  of  silver  beads   . 83 

Sulphides,  assay  of .- 148 

rapid  method  for 149 

reducing  power  of 54,  55,  149 

Sulphuric  acid,  action  of  on  platinum  alloys      157 

Surcharge 173,  188 

Surfusion  of  silver  during  cupellation      82,  93,  94,  98 

T 

Telluride  ores,  losses  in  assay  of 167 

method  of  assay  of 131 

complex 137 

Tellurium,  effect  of,  on  assay 133 

elimination  of  during  cupellation  and  scorifi cation 133 

quantity  of,  in  ores 136 

Temperature  color  scale 102 

Test  lead 31 

Tin  ores,  assay  of 193 

inaccuracies  in 193 

cyanide  assay  for 194 

German  method  of  assay  for 195 

results  obtained  from 196 

Tongs      18,19 

multiple  for  scorifiers 19 

U 

Umpire  assay 40 


INDEX  219 

W 

PAGE 

Weighing 45 

errors  in 172 

Weights,  assay  ton  system  of 51 

conversion  table  for 202 

for  assaying 49 

millieme  system  of 174 

platinum 49 

rider 49 

standardized     .    .    . 50 

Z 

Zinc,  behavior  of,  in  assay 143 

Zinciferous  ores,  assay  of 143 

crucible  method  for 144 

losses  in  assay  of 168 


II 


THE  LIBRARY 
UNIVERSITY  OF  CALIFORNIA 

Santa  Barbara 


THIS  BOOK  IS  DUE  ON  THE  LAST  DATE 
STAMPED  BELOW. 


APR  7    1986 


APR4  1988 


Ill  II  III  II  III  Mill  III  Illl 
AA      000105718    1 


